Implement pacing

[Ralith]

As discussed in the recovery document.

[Ralith]

Of interest is the recently introduced Linux SO_TXTIME cmsg, which allows pacing to be offloaded to the kernel or even network hardware on a correctly configured system.

[Ralith]

An efficient software implementation could work by packets in regular bursts of at most a certain size, rather than attempting to space individual packets out perfectly.

[Ralith]

As of draft 24 this is now required:

Sending multiple packets into the network without any delay between them creates a packet burst that might cause short-term congestion and losses. Implementations MUST either use pacing or limit such bursts to the initial congestion window

[Ralith]

The draft now further requires that bursts be limited specifically to the size of the initial congestion window, and suggests pacing by providing a burst budget that can reach at most the initial congestion window, refills continuously at the pacing rate, and starts full.