Put lcb_t on a diet ;)

lcb_t used a command lookup array for all binary commands
and response codes, but we only use a _SMALL_ subset of those
commands so it doesn't make sense to keep all of them around...

Change-Id: I1662ab88a436e019d9e15034c124c4f3f1d79d9a
Reviewed-on: http://review.couchbase.org/24236
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>