Refactor ChannelMonitor

[ariard]

Basically ChannelMonitor should be split off in 4 components, a trait SecretHolder, a trait TxParser, a trait OnchainTxHandler.

SecretHolder should be combined with the new signer key api (#408) and interface generic enough to support both private mode and watchtowers.

TxParser should be a dumb state machine to parse channel txn topology, it should be easy to support old txn format and new one with anchors.

OnchainTxHandler would get all tx building logic and dynamic-fee handling. If someone wants to implement its own super fancy fee handling logic he should be able to do so.

See for more wip : ariard@94ed2bd