Force-close if closing transaction doesn't confirm in a timely manner

[TheBlueMatt]

t-bast points out at lightning/bolts#894 (comment) that in the case of option_anysegwit someone could use a segwit script with unknown semantics which is nonstandard due to dust output values or some other reason causing the closing transaction to not be confirmable. Thus, we'll need to force-close in these cases. We already expose an API for users to do so, but its kinda annoying to require users do this manually, so we should do it automatically.

[TheBlueMatt]

Basically fixed in #1065.