support to get result response byte size

[HarrisChu]

fix #293

please note:

when invoke GetByteSize(), it would serialize the thrift struct.
if the struct is a huge struct, it may cost much more time and memory.

[Nicole00]

Please keep this issue and PR open, and consider merging it into master if there is a requirement later.

[wenhaocs]

Can we specify it's for debugging and testing purpose only, i.e., add TEST or DEBUG in the function name?