Use syncheight instead of startheight for broadcasting block invs

No need to use the outdated height when we have a more up to date height we can use.
I contentiously removed the "- 2000", which I know will get objections. In fact, there's probably a better way to determine whether to broadcast an inv than using the height number - i.e. broadcast if their height is lower OR if their latest block is not in our best chain.