Add SASL related info to lcb_cntl

This creates a new version of MEMDNODE_INFO which returns the effective
SASL method, as well as a FORCE_SASL_MECH command which will fail the
server if it cannot utilize the user-defined auth method

Change-Id: I292218164cc5482ebb45e791f2c710b50a78b312
Reviewed-on: http://review.couchbase.org/29688
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>