Add TCP keepalive to backend connections
Add TCP keepalive to backend connections

The current backend pooling algorithm seems to be not reusing existing
connections before the max_backend_connections is reached. Before that
is fixed, we need TCP keepalive to make test suite work in environment
like GitHub Action + AWS RDS where TCP connections are likely proxied
and dropped silently without RST after being idle for a while.

edgedb/edgedb,edgedb/edgedb,edgedb/edgedb