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).

Changes:

    Simplify nested complexity
    If an if blocks ends with a return statement then remove the else nesting.

Most of the changes has also been reported in golint https://goreportcard.com/report/github.com/ethereum/go-ethereum#golint

This PR adds service value measurement statistics to the light client. It
also adds a private API that makes these statistics accessible. A follow-up
PR will add the new server pool which uses these statistics to select
servers with good performance.

This document describes the function of the new components:
https://gist.github.com/zsfelfoldi/3c7ace895234b7b345ab4f71dab102d4



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

* les: separate peer into clientPeer and serverPeer

* les: address comments

* les: wait all task routines before drop the peer

* les: address comments

* les: fix issue

les: handler separation

This change

- implements concurrent LES request serving even for a single peer.
- replaces the request cost estimation method with a cost table based on
  benchmarks which gives much more consistent results. Until now the
  allowed number of light peers was just a guess which probably contributed
  a lot to the fluctuating quality of available service. Everything related
  to request cost is implemented in a single object, the 'cost tracker'. It
  uses a fixed cost table with a global 'correction factor'. Benchmark code
  is included and can be run at any time to adapt costs to low-level
  implementation changes.
- reimplements flowcontrol.ClientManager in a cleaner and more efficient
  way, with added capabilities: There is now control over bandwidth, which
  allows using the flow control parameters for client prioritization.
  Target utilization over 100 percent is now supported to model concurrent
  request processing. Total serving bandwidth is reduced during block
  processing to prevent database contention.
- implements an RPC API for the LES servers allowing server operators to
  assign priority bandwidth to certain clients and change prioritized
  status even while the client is connected. The new API is meant for
  cases where server operators charge for LES using an off-protocol mechanism.
- adds a unit test for the new client manager.
- adds an end-to-end test using the network simulator that tests bandwidth
  control functions through the new API.