MB-11755 Receive memcached messages in bulk

Receive worker in DCP client handled memcached messages
individually, reading header first and any extra bytes
as indicated in header. This resulted in making several
thousands of tcp receive calls per design doc and
resulted in timeouts while handling stat responses from
memcached. This behavior is modified to receive all the
available data for memcached from socket and process each
individual response from the stored binary, which results
in far few tcp receive calls. Socket connection is made
active and buffer size is set to 64K, same as TCP window
size. More than 2x increase in speed is observed for
accesses to stat responses from memcached.

Change-Id: I5b25f3ed15b6825b8e88fac6e1eb4d1e5dbe3c5f
Merge commit 'refs/changes/70/40570/9' of ssh://review.couchbase.org:29418/couchdb