core, miner, tests: renamed state methods

* Update => SyncIntermediate
* Added SyncObjects

SyncIntermediate only updates whatever has changed, but, as a side
effect, requires much more disk space.

SyncObjects will only sync whatever is required for a block and will not
save intermediate state to disk. As drawback this requires more time
when more txs come in.

tests/block_test_util.go

@@ -215,7 +215,7 @@ func (t *BlockTest) InsertPreState(ethereum *eth.Ethereum) (*state.StateDB, erro
 		}
 	}
 	// sync objects to trie
-	statedb.Update()
+	statedb.SyncObjects()
 	// sync trie to disk
 	statedb.Sync()