Read should return partial data when available

[prashantv]

Right now, Read will try to fill the whole buffer, even if it means blocking and waiting for another packet.

See PR for more information:
And specific commit to fix this: #173
fb2ea23

I have started a separate branch with a slightly different fix, but there's a few items to do:

• add unit tests to catch this issue and ensure it's fixed
• Consider simplifying Read even more

This doesn't affect any of our users currently as streaming is not widely used, and the few streaming users we have use Thrift streaming, which will pass in the exact buffer size (e.g., the caller knows exactly how much data to read, and wants full reads).