* consensus/ethash: start remote ggoroutine to handle remote mining
* consensus/ethash: expose remote miner api
* consensus/ethash: expose submitHashrate api
* miner, ethash: push empty block to sealer without waiting execution
* consensus, internal: add getHashrate API for ethash
* consensus: add three method for consensus interface
* miner: expose consensus engine running status to miner
* eth, miner: specify etherbase when miner created
* miner: commit new work when consensus engine is started
* consensus, miner: fix some logics
* all: delete useless interfaces
* consensus: polish a bit
* cmd, consensus, eth: split ethash related config to it own
* eth, consensus: minor polish
* eth, consenus, console: compress pow testing config field to single one
* consensus, eth: document pow mode
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.
