blockmanager: address filter header tip lock inconsistency
blockmanager: address filter header tip lock inconsistency

There was a previous assumption that filterHeaderTip and
filterHeaderTipHash were only being accessed within the cfHandler
goroutine. This is no longer the case, as we require the filter header
tip in order to deliver a backlog of notifications for block
notification subscribers.

We also establish a locking order when needing to acquire both
newHeadersMtx and newFilterHeadersMtx in certain cases.

lightninglabs/neutrino