Fix Redis reconnection logic (matrix-org#7482)

Proactively send out `POSITION` commands (as if we had just received a `REPLICATE`) when we connect to Redis. This is important as other instances won't notice we've connected to issue a `REPLICATE` command (unlike for direct TCP connections). This is only currently an issue if master process reconnects without restarting (if it restarts then it won't have written anything and so other instances probably won't have missed anything).

Author: erikjohnston

changelog.d/7482.bugfix

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+Fix Redis reconnection logic that can result in missed updates over replication if master reconnects to Redis without restarting.