Provide lcb_cntl interface

Change-Id: I33111a456d9eda0f7f71e944398302af7f864e5d
Reviewed-on: http://review.couchbase.org/27661
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>