Access signed local commitment through OnchainTxHandler

Implementing dynamic fee bumping implied to cache transaction material
including its witness, to generate a bumped version if needed.

ChannelMonitor is slowly rescoped to its parsing function with ongoing
patchset and data duplicata are removed. If signed local commitment tx
access is needed, it's done through OnchainTxHandler extended API

For test framework purpose, we use the test-only method
ChannelMonitor::unsafe_get_latest_local_commitment_txn to intentionally
generate unsafe local commitment to exerce revocation logic.