[server] dynamically close idle database connections When pool hasn't been empty for `idle_timeout` time, start closing connections.

[server] implement dynamic database pooler Opening too much database connection has a cost at startup and also PostgreSQL as a maximum number of connection (100 by default). This get worse when starting multiple wsgi processes, since each process has its own database pool. Instead of opening `connections-pool-size` connections to the database at startup, just open one and open more only when needed.