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

core/types: fix test for TransactionsByPriceAndNonce

ethash: fix typo

cmd/puppeth: use dumb textual IP filtering

miner: fix typo

cmd: Added support for downloading to another DB instance

* eth/fetcher: check the origin of filter task

* eth/fetcher: add some details to fetcher logs

If the file already existed, the WriteResponse.Size was being set
as the length of the entire file, not just the amount that was
written to the existing file.

Fixes #15216

The accountCache contains a file cache, and remembers from
scan to scan what files were present earlier. Thus, whenever
there's a change, the scan phase only bothers processing new
and removed files.