Handle fallible per commitment point in channel establishment

[alecchendev]

Handles fallible get_per_commitment_point signer method (except for channel reestablish).

We make get_per_commitment_point return a result type so that in the Err case, we (LDK) can resume operation while an async signer fetches the commitment point. This will typically block sending out a message, but otherwise most state updates will occur. When the signature arrives, the user is expected to call ChannelManager::signer_unblocked and we will retry get_per_commitment_point however this time the signer will return the commitment point with Ok, and we'll send out our message.

This PR implements this flow for channel establishment, i.e. open_channel, accept_channel, and channel_ready.

Rough outline of how our HolderCommitmentPoint advances and gets new signatures:

• sending open_channel - creates new outbound channel, immediately requests the first commit point, waits to have the first 2 commit points to be unblocked to send the message
• sending accept_channel - same ^
• sending funding_created - no op regarding commit points
• receiving funding_created - uses point to verify first commitment, then advances commitment, requesting next point
• sending funding_signed - no op
• receiving funding_signed - same as above, verifies, advances, requests next point
• sending channel_ready - waits to have the next commit point ready, then once unblocked sends the current point in the channel_ready message