travis: bump Android builder to Go 1.13.6

appveyor: bump Go to 1.13.6 on Windows

* trie/tests: add benchmarks and update trie tests

* trie: update benchmark tests

* trie: utilize callbacks instead of amassing lists of hashes in database ref/unref

* trie: replace remaining non-callback based accesses

* core: write chain data in atomic way

* core, light: address comments

* core, light: fix linter

* core, light: address comments

This change works around the 32k RDATA character limit per change
request and fixes several issues in the deployer which prevented it from
working for our production trees.

This is a temporary fix for a problem which started happening when the
dialer was changed to read nodes from an enode.Iterator. Before the
iterator change, discovery queries would always return within a couple
seconds even if there was no Internet access. Since the iterator won't
return unless a node is actually found, discoverTask can take much
longer. This means that the 'emergency connect' logic might not execute
in time, leading to a stuck node.

* p2p: wait for goroutine exit, fixes #20558

* p2p: wait for all slots on exit

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

* build: upgrade golangci to 1.22.2

* .golangci.yml: don't fail on asset deadcode

travis, build: enable Ubuntu Focal and Go 1.13.6 on PPA

* build: put GOPATH in /tmp on launchpad

* build: don't remove GOPATH from go tool environment

* build: remove env.sh

This removes the dirty symlink-to-self hack we've had for years. The
script was added to enable building without GOPATH and did that job
reliably for all this time. We can remove the workaround because modern
Go supports building without GOPATH natively.

* Makefile: add GO111MODULE=on to environment

core: set max tx size down to 2 slots (64KB)

eth: check propagated block malformation on receiption

* tests for tx size

* alow multiple slots transactions

* tests for tx size limit (32 KB)

* change tx size tests to use addRemoteSync instead of validateTx (requested in pool request).

* core: minor tx slotting polishes, add slot tracking metric

Co-authored-by: Michael Riabzev <RiabzevMichael@gmail.com>
Co-authored-by: Péter Szilágyi <peterke@gmail.com>

core/state: add more verbosity to panic

This reverts commit a1bc0e3c.

* internal/testlog: print file+line number of log call in test log

This changes the unit test logger to print the actual file and line
number of the logging call instead of "testlog.go:44".

Output of 'go test -v -run TestServerListen ./p2p' before this change:

    === RUN   TestServerListen
    --- PASS: TestServerListen (0.00s)
        testlog.go:44: DEBUG[01-08|15:16:31.651] UDP listener up         addr=127.0.0.1:62678
        testlog.go:44: DEBUG[01-08|15:16:31.651] TCP listener up         addr=127.0.0.1:62678
        testlog.go:44: TRACE[01-08|15:16:31.652] Accepted connection     addr=127.0.0.1:62679

And after:

    === RUN   TestServerListen
    --- PASS: TestServerListen (0.00s)
        server.go:868: DEBUG[01-08|15:25:35.679] TCP listener up         addr=127.0.0.1:62712
        server.go:557: DEBUG[01-08|15:25:35.679] UDP listener up         addr=127.0.0.1:62712
        server.go:912: TRACE[01-08|15:25:35.680] Accepted connection     addr=127.0.0.1:62713

* internal/testlog: document use of t.Helper

Update copyright from 2013-2019 to 2013-2020

* add node.go unit test file node_test.go

* add node_test.go file license and rollback trie_test.go

* fix unused variable v

* trie: fix license year

Co-authored-by: Péter Szilágyi <peterke@gmail.com>

* les: move the checkpoint oracle into its own package

It's first step of refactor LES package. LES package
basically can be divided into LES client and LES server.
However both sides will use checkpoint package for
status retrieval and verification. So this PR moves
checkpoint oracle into a separate package

* les: address comments

Fixes #19024
Fixes #19332

This commit intents to replicate the DialHTTPWithClient function which allows
creating a RPC Client using a custom dialer but for websockets.

We introduce a new DialWebsocketWithDialer function which allows the caller
to instantiate a new websocket client using a custom dialer.