Use HikariCP for DB connection pool

[rads]

In a real app you'll probably end up using HikariCP to reduce the latency of your DB queries. This requires a bit of refactoring to replace the existing db usage with a middleware to make sure we only initialize the connection pool once when the server starts.

I have a branch here that I can extract into a separate PR if you're interested:
master...rads:todo-backend-reitit:rain