fundingmanager: hold mutex during ctx cancellation
fundingmanager: hold mutex during ctx cancellation

This commit changes cancelReservationCtx to gold the resMtx from start
to finish. Earlier it would lock at different times only when accessing
the maps, meaning that other goroutines (I'm looking at you
PeerTerminationWatcher) could come in and grab the context in between
locks, possibly leading to a race.

aakselrod/lnd,LightningNetwork/lnd,aakselrod/lnd,LightningNetwork/lnd