Add forwarding gather() function to IOBufQueue

Add forwarding gather() function to IOBufQueue

Summary:
I'm working with a parser that requires a certain number of
contiguous bytes to be able to make forward progress. I'm also using
IOBufQueue to receive data from an AsyncReader::ReadCallback with
pre/postallocate. So, I need to call gather() to ensure that the queue's
front IOBuf has the right number of contiguous bytes available.

Reviewed By: djwatson

Differential Revision: D3838079

fbshipit-source-id: 9f1ec5c86895eb1b2b109f9f145ca42d2dbba4c6