blockmanager: introduce new signals for blockheader/filterheader tip changes
blockmanager: introduce new signals for blockheader/filterheader tip changes

In this commit, we introduce a new set of signals for block and filter
header tip changes. These signals will be used by sub-systems to be
notified of whenever the tip for a particular chain cahgnes. This will
be useful to allow the sub-system to be notified w.r.t when it's ok for
them to start their actual task based on the current block height.

This will replace all the polling, and also the startCFHeaderSync struct
as well. We don't use the block subscriptions for this as these callers
don't require knowledge of each new attached header, instead they only
need to know when the actual tip changes.

lightninglabs/neutrino