github.com/stretchr/testify is a useful library for doing
assertion in tests. It makes assertions in test more less verbose and
more comfortable to read and use.

travis: bump Android NDK version and Android Go builder

* build: [finishes #15372] implements generalized linter and travis job

* .travis, build: minor polishes, disable deadcode

All logs in the FilterLog return value would be the same object 
because the for loop captured the pointer to the iteration variable.

* core: allow price bump at threshold

* core: test changes to allow price bump at threshold

* core: reinstate tx replacement test underneath threshold

* core: minor test failure message cleanups

core, swarm: typo fixes

* les: fix topic ID

* core/bloombits: fix interface conversion

cmd/rlpdump: allow hex input to have leading '0x'

* core, eth, les: fix messy code

* les: fixed tx status test and rlp encoding

* core: add a workaround for light sync

This PR implements the new LES protocol version extensions:

* new and more efficient Merkle proofs reply format (when replying to
  a multiple Merkle proofs request, we just send a single set of trie
  nodes containing all necessary nodes)
* BBT (BloomBitsTrie) works similarly to the existing CHT and contains
  the bloombits search data to speed up log searches
* GetTxStatusMsg returns the inclusion position or the
  pending/queued/unknown state of a transaction referenced by hash
* an optional signature of new block data (number/hash/td) can be
  included in AnnounceMsg to provide an option for "very light
  clients" (mobile/embedded devices) to skip expensive Ethash check
  and accept multiple signatures of somewhat trusted servers (still a
  lot better than trusting a single server completely and retrieving
  everything through RPC). The new client mode is not implemented in
  this PR, just the protocol extension.

* cmd, consensus, core, miner: instatx clique for --dev

* cmd, consensus, clique: support configurable --dev block times

* cmd, core: allow --dev to use persistent storage too

core: fire tx event on replace, expand tests

build: start shipping Ubuntu Artful Aardvark binaries

cmd/faucet: support twitter, google+ and facebook auth too

core/vm: check opcode stack before readonly enforcement

trie: make hasher benchmark meaningful post-caches