Common API to communicate with PostgreSQL server.

JSON read & write shortcuts.

A dedicated namespace with native (built-in) Postgres OID types.

A simple connection pool. Runs open connections in two
data structures: a queue of free connections and a map
of busy connections. The connections are taken from a tail
of the queue and put back into the head.

Every time the connection is borrowed, it's check for expiration.
An expired connection gets closed, and the one is produced.

Should all free connections are in use at the moment and the client
is going to borrow another one, an exception is triggered.

When a connection is put back, it's checked for expiration and for
the transaction status. Connections that are in the error state
are closed; Connections that are in a transaction are rolled back.

Like `ring.middleware.json` but faster because uses
internal JSON engine instead of Cheshire.