Handle fallible per commitment point for RAA

[alecchendev]

Builds on #3149.

This is the first PR to handle async get_per_commitment_point, and only handles the Err cases for retrieving revoke_and_ack. We still need to handle this during funding (#3109) and channel reestablish (no PR yet) in upcoming PRs.

For all async signing, we try to go about our normal business, and when we fail to get a signature from our signer, we pause our channel, and only unpause upon the user calling ChannelManager::signer_unblocked. With the signer_pending_revoke_and_ack flag being added in the prereq PR, we simply set the flag and allow get_last_revoke_and_ack to return an Option. We also make sure that in cases where we must send RAA then CS, that we defer sending a CS (even if it's available) until the signature for the RAA is unblocked.