Make acces and signature of local commitment transaction unique

Local commitment transaction broadcast can be triggered by a)
a Channel force-close or b) reaching some block height implying
a onchain HTLC-timeout. If one of this condition is fulfilled,
commitment is signed and from then any state update would be
rejected.

ChannelMonitor init at Channel creation need to be refactored
before to make get_fully_signed_local_tx infaillible to avoid
choking in the test framework.