Implement retries on initial database connection

[kenellorando]

The Cadence container starts up faster than the Postgres container it depends on when launched through a Docker Compose stack normally. Since Cadence does a bunch of database configuration on initial connect, we end up without a working database.

We alleviate this currently with a hardcoded wait time before initial connect.

cadence/cadence/server/db_postgres.go

Lines 22 to 24 in baa6f2b

	// We wait a bit to give some leeway for Postgres to finish startup.
	// Obligatory: There's probably a better way to do this.
	time.Sleep(5 * time.Second)

A more precise method (maybe timed auto-retry through Compose or in code) would be better.