p2p/discover: fix nil-dereference due to race

* move error variable to error.go

* Update error.go

Edit "Genesis" to "genesis"

* core: fix import errors on clique crashes + empty blocks

* cosensus/clique, core: add test for the mirrored state issue

* core: address todo question wrt log count

* core: raise a louder warning for non-clique known blocks

swarm v0.4-rc1

swarm/storage: remove traces for put/get/set (#1389)

* swarm/storage: remove traces for put/get/set

* swarm/storage: remove Has traces

swarm/storage/localstore: add gc metrics, disable flaky test

swarm/storage/localstore: fix broken metric (#1373)

p2p/protocols: count different messages (#1374)

cmd/swarm: disable snapshot create test due to constant flakes (#1376)

swarm/network: remove redundant goroutine (#1377)

swarm/network: add want delay timer to syncing (#1367)

swarm/network: synchronise peer.close() (#1369)

swarm/api: integrate tags to count chunks being split and stored
swarm/api/http: integrate tags in middleware for HTTP `POST` calls and assert chunks being calculated and counted correctly
swarm: remove deprecated and unused code, add swarm hash to DoneSplit signature, remove calls to the api client from the http package

* swarm/chunk: add tags backend to chunk package

swarm/network: measure how many chunks a node delivers (#1358)

* swarm/shed: remove metrics fields from DB struct

* swarm/schunk: add String methods to modes

* swarm/storage/localstore: add metrics and traces

* swarm/chunk: unknown modes without spaces in String methods

* swarm/storage/localstore: remove bin number from pull subscription metrics

* swarm/storage/localstore: add resetting time metrics and code improvements

cmd/swarm/swarm-smoke: improve smoke tests (#1337)

swarm/network: remove dead code (#1339)

swarm/network: remove FetchStore and SyncChunkStore in favor of NetStore (#1342)

 cmd/faucet: embed git commit hash/date, disable Google+

* add-date-to unstable

* fields-insteadof-split

* internal/build: support building with missing git

* docker: add git history back to support commit date in version

* internal/build: use PR commits hashes for PR builds

* core: import known blocks if they can be inserted as canonical blocks

* core: insert knowns blocks

* core: remove useless

* core: doesn't process head block in reorg function

eth/downloader: fix header delays during chain dedup

* accounts: add note about backing up the keystore

* cmd, accounts: move the printout to accountCreate

* internal, signer: add info when new account is created via rpc

* cmd, internal, signer: split logs

* cmd/geth: make account new output a bit more verbose

* core: add tests for canonicality confusion

* core: delete stale future canon number mappings during reorg to shorter+heavier chain

* trie: fix merkle proof

* trie: use hasher instead of allocate keccack256 every time

* trie: add comments

p2p/discover: split out discv4 code

This change makes getBalance, getCode, getStorageAt, getProof,
call, getTransactionCount return an error if the block number in
the request doesn't exist. getHeaderByNumber still returns null
for missing headers.

* consensus,core,miner: avoid overhead of creating a new block

* consensus: nitpick dot

* consensus: fix some comment formatting nits

This change restructures the internals of p2p/discover to make room for
the discv5 code which will soon be added to this package.

- packet type names now have a "V4" suffix.
- ListenUDP returns *UDPv4 instead of *Table. This technically breaks
  the API but the only caller in go-ethereum is package p2p, which uses
  a compatible interface and doesn't need changes.
- The internal transport interface is changed to make Table reusable for v5.
- The 'lookup' code moves from table to transport. This required
  updating the lookup unit test to use udpTest instead of a custom transport.