core, miner: miner header validation, transaction & receipt writing

* Miners do now verify their own header, not their state.
* Changed old putTx and putReceipts to be exported
* Moved writing of transactions and receipts out of the block processer
  in to the chain manager. Closes #1386
* Miner post ChainHeadEvent & ChainEvent. Closes #1388

core/manager.go

@@ -14,5 +14,6 @@ type Backend interface {
 	TxPool() *TxPool
 	BlockDb() common.Database
 	StateDb() common.Database
+	ExtraDb() common.Database
 	EventMux() *event.TypeMux
 }