Add keep-alive support to the HTTP module.

Add keep-alive support to the HTTP module.
Requests are split away from connections. When a request is received try
to reuse an IDLE connection. If none is around start a new one (unless
there are too many connections inflight).

Idle connections are kept for 10sec and closed after that time. For
rpki-client this is plenty of time since RRDP exchanges will be a burst
of requests. So the connection used to fetch the notification XML file
will be reused to fetch all delta XML files.

This reduces the CPU load since far less TLS handshakes need to happen.

OK job@ deraadt@