Revert "agent: use a dedicated connection for pubsub"

This reverts commit c02a459662a93b752d6dc55bec701825492901ad. We get a
pool of connections, not a connection. There is no need to have several
of them.
redis: use a greater idle timeout

By default, the timeout is 30 seconds. This is too short for pub/sub in
case nothing is published in the timeframe. Use an hard-coded value of
10 minutes.