The original idea behind this change was to remove a use of the
deprecated CancelRequest method. Simply removing it would've been an
option, but I couldn't resist and did a bit of a refactoring instead.
All remote sealing code was contained in a single giant function. Remote
sealing is now extracted into its own object, remoteSealer.
* 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
