Adding support sumbit_multi operation

[vetss]

For supporting of sumbit_multi operation I have created 2 pull requests (into master and netty4 branches):
fizzed#3
fizzed#4

Can you please check them and merge into the repo code.

PS: The pull request fizzed#4 shows me some error in unit tests. I have tried to reproduce the error locally several times but all tests pass at my side. Can we retest it ones more ?