This PR has two changes in the les protocol:

- the auxRoot is not supported. See ethereum/devp2p#171 for more information
- the empty response will be returned in GetHelperTrieProofsMsg request if the merkle
   proving is failed. note, for backward compatibility, the empty merkle proof as well as
   the request auxiliary data will still be returned in  les2/3 protocol no matter the proving
   is successful or not. the proving failure can happen e.g. request the proving for a
   non-included entry in helper trie (unstable header).

This commit splits the eth package, separating the handling of eth and snap protocols. It also includes the capability to run snap sync (https://github.com/ethereum/devp2p/blob/master/caps/snap.md

) , but does not enable it by default. 

Co-authored-by: Marius van der Wijden <m.vanderwijden@live.de>
Co-authored-by: Martin Holst Swende <martin@swende.se>

* les: introduce forkID

* les: address comment

This adds a few tiny fixes for les and the p2p simulation framework:

LES Parts

- Keep the LES-SERVER connection even it's non-synced

  We had this idea to reject the connections in LES protocol if the les-server itself is
  not synced. However, in LES protocol we will also receive the connection from another
  les-server. In this case even the local node is not synced yet, we should keep the tcp
  connection for other protocols(e.g. eth protocol).

- Don't count "invalid message" for non-existing GetBlockHeadersMsg request

  In the eth syncing mechanism (full sync, fast sync, light sync), it will try to fetch
  some non-existent blocks or headers(to ensure we indeed download all the missing chain).
  In this case, it's possible that the les-server will receive the request for
  non-existent headers. So don't count it as the "invalid message" for scheduling
  dropping.

- Copy the announce object in the closure

  Before the les-server pushes the latest headers to all connected clients, it will create
  a closure and queue it in the underlying request scheduler. In some scenarios it's
  problematic. E.g, in private networks, the block can be mined very fast. So before the
  first closure is executed, we may already update the latest_announce object. So actually
  the "announce" object we want to send is replaced.

  The downsize is the client will receive two announces with the same td and then drop the
  server.

P2P Simulation Framework

- Don't double register the protocol services in p2p-simulation "Start".

  The protocols upon the devp2p are registered in the "New node stage". So don't reigster
  them again when starting a node in the p2p simulation framework

- Add one more new config field "ExternalSigner", in order to use clef service in the
  framework.

* les: move NodeStateMachine from clientPool to LesServer

* les: new header broadcaster

* les: peerCommons.headInfo always contains last announced head

* les: remove clientPeerSet and serverSet

* les: fixed panic

* les: fixed --nodiscover option

* les: disconnect all peers at ns.Stop()

* les: added comments and fixed signed broadcasts

* les: removed unused parameter, fixed tests

* les, les/lespay/server: refactor client pool

* les: use ns.Operation and sub calls where needed

* les: fixed tests

* les: removed active/inactive logic from peerSet

* les: removed active/inactive peer logic

* les: fixed linter warnings

* les: fixed more linter errors and added missing metrics

* les: addressed comments

* cmd/geth: fixed TestPriorityClient

* les: simplified clientPool state machine

* les/lespay/server: do not use goroutine for balance callbacks

* internal/web3ext: fix addBalance required parameters

* les: removed freeCapacity, always connect at minCapacity initially

* les: only allow capacity change with priority status

Co-authored-by: rjl493456442 <garyrong0905@gmail.com>

* les: close all connected les-server when shutdown

* les: linter nitpick

Co-authored-by: Martin Holst Swende <martin@swende.se>

* cmd, consensus, eth, les: implement light fetcher

* les: address comment

* les: address comment

* les: address comments

* les: check td after delivery

* les: add linearExpiredValue for error counter

* les: fix import

* les: fix dead lock

* les: order announces by td

* les: encapsulate invalid counter

* les: address comment

* les: add more checks during the delivery

* les: fix log

* eth, les: fix lint

* eth/fetcher: address comment

* les: drop the message if the entire p2p connection is stuck

* les: fix lint

* les: separate peer into clientPeer and serverPeer

* les: address comments

Co-authored-by: ligi <ligi@ligi.de>

* les: detailed relative cost metrics

* les: filter txpool relative request statistic

* les: initialize price factors

* les: increased connected bias to lower churn rate

* les: fixed clientPool.setLimits

* core: do not use mutex in GetAncestor

* les: bump factor db version again

* les: add metrics

* les, light: minor fixes

* les: wait all task routines before drop the peer

* les: address comments

* les: fix issue

les: handler separation