Fix deadlock in handle_error!() when we have HTLCs to fail-back.

Fix deadlock in handle_error!() when we have HTLCs to fail-back.

This partially reverts 933ae3470309f21ef7537ffbcdc42070d60e1e74,
though note that 933ae3470309f21ef7537ffbcdc42070d60e1e74 fixed a
similar deadlock while introducing this one.

If we have HTLCs to fail backwards, handle_error!() will call
finish_force_close_channel() which will attempt to lock channel_state
while it is locked at the original caller. Instead, hold the lock for
shorter scopes such that it is not held upon entering handle_error!().

Co-authored-by: Matt Corallo <git@bluematt.me>
Co-authored-by: Jeffrey Czyz <jkczyz@gmail.com>