* core: use a wrapped `map` and `sync.RWMutex` for `TxPool.all` to remove contention in `TxPool.Get`.

* core: Remove redundant `txLookup.Find` and improve comments on txLookup methods.

* 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, 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.

* core: Fix flaw where underpriced locals were removed

* core: minor code cleanups for tx pool tests

The commit reworks the transaction pool queue limitation tests
to cater for testing local accounts, also testing the nolocal flag.

In addition, it also fixes a panic if local transactions exceeded
the global queue allowance (no accounts left to drop from) and also
fixes queue eviction to operate on all accounts, not just the one
being updated.