postgres connector: replace pgxpool.Pool with pgx.Conn

[serprex]

We currently create a pool per connector. This is not generally useful. Connection pools are best when you have one pool for the entire application, such as what we do for the catalog pool. Change connector to use pgx.Conn as connectors should not generally be used in a multithreaded way

In situations where you need a 2nd connection the pool would end up creating that 2nd connection, & in most scenarios that 2nd connection should be short lived (eg, 5 minute slot info checks) instead of having them idle most of the time

[iskakaushik]

for connections established by PeerDB, can we also set application_name to peerdb as a part of this.