Initial Block Download optimisations #5099

I've been developing these improvements in parallel with headersfirst8.

It's now been optimised for use with early to late block chain so adapts to the block sizes and also the bandwidth of each node to make optimal use of the connections available.

The sipa disconnect code has been disabled as it usually disconnected the most important node (with my changes), but it will still display when the logic would have executed.

Please do test and compare download times with before this test and I'm sure you'll see improvements.

It's not yet optimised for use with earlier blocks (which were small enough to download in batches of 500 each), but that will be coming shortly. Raising the pull request so that people can view and comment on some of the changes.

There are probably some commits in here that can be removed. I've kept it fairly granular so that some of the changes are easily optional. The big change is the commit called "Track partial download progress...", and I've recently updated this so that it now changes the window size per node based upon the bandwidth each note is achieving. @laanwj Is it worth tagging members of the core development team in a comment to draw their attention to this?

@rebroad Possibly. But it could be that this list of change is too intimidating for one pull, and it may be better to trickle the changes instead of having one umbrella "Headersfirst8 improvements".

tumbleweed

I don't know what travis is talking about - it seems to be running an old version that isn't present in the current branch.

Needs rebase.