Switch to JDBC + virtual threads

Switch db internals to use jdbc instead of jasync postgresql. Reason for this is that jasync seems to be no longer maintained and looks like it would need some work to adapt to recent changes in netty.

JDBC is well supported and by using virtual threads, we get most of the same advantages of using non blocking IO. Additionally, it simplifies connection management and transactionality.

The API stays the same; the only change needed is you now need to initialize your doc store with a HikariCP datasource for postgresql.