Fix edge-case when a module client is unblocked (#8618)

Scenario:
1. A module client is blocked on keys with a timeout
2. Shortly before the timeout expires, the key is being populated and signaled
   as ready
3. Redis calls moduleTryServeClientBlockedOnKey (which replies to client) and
   then moduleUnblockClient
4. moduleUnblockClient doesn't really unblock the client, it writes to
   server.module_blocked_pipe and only marks the BC as unblocked.
5. beforeSleep kics in, by this time the client still exists and techincally
   timed-out. beforeSleep replies to the timeout client (double reply) and
   only then moduleHandleBlockedClients is called, reading from module_blocked_pipe
   and calling unblockClient

The solution is similar to what was done in moduleTryServeClientBlockedOnKey: we
should avoid re-processing an already-unblocked client