Construct all ChannelMonitor mutexes in the same function

Construct all ChannelMonitor mutexes in the same function

When we add lockorder detection based on mutex construction site
rather than mutex instance in the next commit, ChannelMonitor's
PartialEq implementation causes spurious failures. This is caused
by the lockorder detection logic considering the ChannelMonitor
inner mutex to be two distinct mutexes - one when monitors are
deserialized and one when monitors are created fresh. Instead, we
attempt to tell the lockorder detection logic that they are the
same by ensuring they're constructed in the same place - in this
case a util method.