This change

- implements concurrent LES request serving even for a single peer.
- replaces the request cost estimation method with a cost table based on
  benchmarks which gives much more consistent results. Until now the
  allowed number of light peers was just a guess which probably contributed
  a lot to the fluctuating quality of available service. Everything related
  to request cost is implemented in a single object, the 'cost tracker'. It
  uses a fixed cost table with a global 'correction factor'. Benchmark code
  is included and can be run at any time to adapt costs to low-level
  implementation changes.
- reimplements flowcontrol.ClientManager in a cleaner and more efficient
  way, with added capabilities: There is now control over bandwidth, which
  allows using the flow control parameters for client prioritization.
  Target utilization over 100 percent is now supported to model concurrent
  request processing. Total serving bandwidth is reduced during block
  processing to prevent database contention.
- implements an RPC API for the LES servers allowing server operators to
  assign priority bandwidth to certain clients and change prioritized
  status even while the client is connected. The new API is meant for
  cases where server operators charge for LES using an off-protocol mechanism.
- adds a unit test for the new client manager.
- adds an end-to-end test using the network simulator that tests bandwidth
  control functions through the new API.

* les, light: fix CHT trie retrievals

* les, light: minor polishes, test remote CHT retrievals

* les, light: deterministic nodeset rlp, bloombits test skeleton

* les: add an event emission to the les bloombits test

* les: drop dead tester code

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.

* les: implement request distributor, fix blocking issues
* core: moved header validation before chain mutex lock

go get github.com/gordonklaus/ineffassign
ineffassign .