Fix tightloop over connecting to replication server

If the client failed to process incoming commands during the initial set
up of the replication connection it would immediately disconnect and
reconnect, resulting in a tightloop.

This can happen, for example, when subscribing to a stream that has a
row that is too long in the backlog.

The fix here is to not consider the connection successfully set up until
the client has succesfully subscribed and caught up with the streams.
This ensures that the retry logic timers aren't reset until then,
meaning that if an error does happen during start up the client will
continue backing off before retrying again.