Add configuration change notification

Currently, clients track configuration changes by listening for NVM packets from the server, in addition to polling the server at a certain rate to ensure that any configuration changes to buckets that are not being touched are received. Polling memcached for configuration changes costs unnecessary network bandwidth in most cases, as the configuration has not actually changed.

The proposed solution to this problem would be to allow clients to subscribe to receiving interleaved configuration change notification packets from the server. Once the client has subscribed to these notifications, whenever memcached receives a new configuration revision from ns_server, it would dispatch a notification to all connected clients who have requested this. The notification should consist of only the new revision ID, the client will be responsible for actually fetching the configuration from the server. This will allow us to improve upon this feature by listening for notifications on a subset of the cluster nodes, and then only perform a single fetch.

Note that because subscription is voluntary, only clients which are capable of handling out-of-order operations would be subscribing (though this describes all clients in the past 2 years or so).