Instead of managing Postgres connections on our own, use the
`with-connection` postmodern function with pooling enabled as a route
decorator.
This should resolve at least some of the issues from b/113 with
leaking connections, and an unreported issue with connections being
reused while transactions are in progress.
Change-Id: I1ed68667a3240900de1ae69df37d2d3018caf204
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5198
Tested-by: BuildkiteCI
Reviewed-by: eta <tvl@eta.st>
Autosubmit: tazjin <tazjin@tvl.su>