Allow shutdown-init in more cases

[TheBlueMatt]

Similar to #2096, if we're disconnected from a peer we should still allow shutdown, we just wont be able to do anything until reconnect. Further, we should similarly allow shutdown even if there's pending local-announced HTLCs, again just without sending the shutdown message until those resolve.

[TheBlueMatt]

Slipping. Important but not so important we need to hold.