1. In lnhtlc, sent_in_ctn and failed_in_ctn need to look at the
remote ctx, and they need to be called when we receive a revocation,
not when we send one.
2. In lnchannel, we use 3 lnworker callbacks:
- payment sent/payment failed (called when we receive a revocation)
- payment received (called when we send a revocation)
3. Make revoke_current_commitment return a single value.
The second value was only used in tests, there is no need
to bloat the code with that
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