* les/lespay/server: fix balance expiration and add test

* les: move client balances to a new db

* les: rename lespayDb to lesDb

* 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>

* 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

* core/state/snapshot: start fixing disk iterator seek

* ethdb, rawdb, leveldb, memorydb: implement iterators with prefix and start

* les, core/state/snapshot: iterator fixes

* all: remove two iterator methods

* all: rename Iteratee.NewIteratorWith -> NewIterator

* ethdb: fix review concerns

* les: separate peer into clientPeer and serverPeer

* les: address comments

* build: use golangci-lint

This changes build/ci.go to download and run golangci-lint instead
of gometalinter.

* core/state: fix unnecessary conversion

* p2p/simulations: fix lock copying (found by go vet)

* signer/core: fix unnecessary conversions

* crypto/ecies: remove unused function cmpPublic

* core/rawdb: remove unused function print

* core/state: remove unused function xTestFuzzCutter

* core/vm: disable TestWriteExpectedValues in a different way

* core/forkid: remove unused function checksum

* les: remove unused type proofsData

* cmd/utils: remove unused functions prefixedNames, prefixFor

* crypto/bn256: run goimports

* p2p/nat: fix goimports lint issue

* cmd/clef: avoid using unkeyed struct fields

* les: cancel context in testRequest

* rlp: delete unreachable code

* core: gofmt

* internal/build: simplify DownloadFile for Go 1.11 compatibility

* build: remove go test --short flag

* .travis.yml: disable build cache

* whisper/whisperv6: fix ineffectual assignment in TestWhisperIdentityManagement

* .golangci.yml: enable goconst and ineffassign linters

* build: print message when there are no lint issues

* internal/build: refactor download a bit

This PR implements the LES server RPC API. Methods for server
capacity, client balance and client priority management are provided.

* les: loose restriction of unit tests

* les: update unit tests

* les, light: slim the unit tests

* les: rework clientpool

* 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