Add CaughtUp() function (needed by later commits)
Improve getheaders logging
debug "block"
Use syncheight instead of startheight for broadcasting block invs
Better logging of stalling
Fix stall timeout
Display error when duplicate block received
Only send initial getheaders when !CaughtUp()
Change check whether to send block inv after UpdateTip
Add height to "Requesting block" debug
Don't getdata blocks as soon as the invs are seen. (wait for the headers instead)
Fix logic for nSyncStarted (only increases when headers actually arrive).