Refactor conn buffer members into struct

Move the various read & write buffer members (buf, curr, size, bytes) into their
own struct. This allows a pointer to whole buffer to be passed around,
simplifying some of the buffer management changes coming for MB-11067.

Change-Id: Iaa8daa83a5dd84e0ffd995b46a69f4c4dc7b5cf6
Reviewed-on: http://review.couchbase.org/38554
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>