Allow connections to "unselect bucket"

Currently memcached connections begin against "no bucket" until the select_bucket command is executed. select_bucket can be used to switch buckets but there's currently no way to "unselect bucket" and get the connection back into a bucketless state.

Such a command would be very useful for clients managing pools of connections where the pool spans buckets. (ns_server is one such client.) As seen in MB-34713, a connection that used to be against one bucket but is about to be used to manage another bucket can be unexpectedly closed at an inconvenient time.