When keyring replication starts, we take a handle to the state store. But
whenever a snapshot is restored, this handle is invalidated and no longer points
to a state store that is receiving new keys. This leaks a bunch of memory too!

In addition to operator-initiated restores, when fresh servers are added to
existing clusters with large-enough state, the keyring replication can get
started quickly enough that it's running before the snapshot from the existing
clusters have been restored.

Fix this by updating the handle to the state store whenever the store's abandon
channel is closed. Refactor the query for key metadata to use blocking queries
for efficiency.

…ing ERR_WAIT block