Add lcb_MULTICMD_CTX for observe

This is a new 'v3' API which allows the stuffing of multiple commands
into a temporary structure, with a final 'done' phase which actually
assembles the packet. It fits well with how OBSERVE works and may be
fitted for endure in a subsequent commit.

Change-Id: If5a6478c49615355ac97bab106e58a19e1312f05
Reviewed-on: http://review.couchbase.org/38859
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>