Nuke the "plain-text" command protocol

Change-Id: I8222c559c1a464b9c358f92628a624ff00c40391
Reviewed-on: http://review.couchbase.org/27798
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>