This commit does various code refactorings:

- generalizes and moves the request retrieval/timeout/resend logic out of LesOdr
  (will be used by a subsequent PR)
- reworks the peer management logic so that all services can register with
  peerSet to get notified about added/dropped peers (also gets rid of the ugly
  getAllPeers callback in requestDistributor)
- moves peerSet, LesOdr, requestDistributor and retrieveManager initialization
  out of ProtocolManager because I believe they do not really belong there and the
  whole init process was ugly and ad-hoc

This commit adds pluggable consensus engines to go-ethereum. In short, it
introduces a generic consensus interface, and refactors the entire codebase to
use this interface.

This commit solves several issues concerning the genesis block:

* Genesis/ChainConfig loading was handled by cmd/geth code. This left
  library users in the cold. They could specify a JSON-encoded
  string and overwrite the config, but didn't get any of the additional
  checks performed by geth.
* Decoding and writing of genesis JSON was conflated in
  WriteGenesisBlock. This made it a lot harder to embed the genesis
  block into the forthcoming config file loader. This commit changes
  things so there is a single Genesis type that represents genesis
  blocks. All uses of Write*Genesis* are changed to use the new type
  instead.
* If the chain config supplied by the user was incompatible with the
  current chain (i.e. the chain had already advanced beyond a scheduled
  fork), it got overwritten. This is not an issue in practice because
  previous forks have always had the highest total difficulty. It might
  matter in the future though. The new code reverts the local chain to
  the point of the fork when upgrading configuration.

The change to genesis block data removes compression library
dependencies from package core.

These accessors were introduced by light client changes, but
the only method that is actually used is GetNumberU64. This
commit replaces all uses of .GetNumberU64 with .Number.Uint64.