Divide the root updating of stateObjects into goroutines if number of stateObjects is at least the threshold
-> results in 1.55 times faster
statedb_test.go/TestIntermediateUpdateConcurrently: add test to check if the states after processed with both options are identical

statedb/intermediateRoot: update options for concurrent: get changes that are not made yet to run concurrently
state_object/updateTrie: add option for concurrency: store and return data waiting for update

…n be run concurrently

statedb.go/StateDB struct: remove lock since it will not be used for concurrent update
statedb.go/IntermediateRoot: check ConcurrentUpdateThreshold for choosing between sequential and concurrent update
state_object.go: change "concurrent" variable to a more reasonable name "skipTrieUpdate"
flags.go/MakeChain: add flag when creating a chain