MB-7960: handling unsupported select_bucket cmd in XMEM

If select_bucket command is no longer supported, we can safely
ignore the error msg returned from remote memcached when issuing
"select_bucket" command.

Change-Id: I4df3dc2ceb6a58f1b9afcfbe418925615e69ff33
Reviewed-on: http://review.couchbase.org/28040
Reviewed-by: Junyi Xie <junyi.couchbase@gmail.com>
Tested-by: Junyi Xie <junyi.couchbase@gmail.com>