more aggressive speed tuning when bandwidth is available

[Antoine Martin]

If we have network bandwidth to spare, then we should push the speed auto tuning higher to make use of it, giving us a higher fps when we can sustain it.

Testing using a video player on a LAN, I got the best results using "--speed=100" (x264 "ultrafast") which consumed a lot of bandwidth at 1080p (peaks at 70Mbps) but when the bandwidth is available we should make use of it. (unless we decide to honour bandwidth constraints, see #417)

[Antoine Martin]

Too late in the release cycle to make this sort of somewhat risky change - re-scheduling.

[Antoine Martin]

Some minimal changes were made in 0.11 to ensure we adjust the speed a little bit better, see r5206

[Antoine Martin]

A number of changes have been made to try to keep the encoder more busy, notably: r5339 + r5340, r5299, this will do for now. (see also #502 and the receive buffer size changes)