Check best block before connecting a new one in `ChainMonitor`

[TheBlueMatt]

Similar to #987, but I suspect made much worse by #2528 - if we get a ChannelMonitor from the ChannelManager it may (or may not) have the same best-block as the rest of the monitors in ChainMonitor. Thus, we need to handle it somehow and make sure things get to the same state when adding a new monitor. I think the easiest way to do this for now is to say that ChainMonitor has to get block connection events after ChannelManager, and check best block before connecting in the ChainMonitor loop.

Tagging 117 as I'm afraid this may be much more likely and we should avoid regressing here.

[jbesraa]

@TheBlueMatt when is .117 scheduled to be released?

[wpaulino]

By the end of the month, but I think @G8XSU / @arik-so may already be working on this.