neutrino: make ChainService.IsCurrent use IsFullySynced
neutrino: make ChainService.IsCurrent use IsFullySynced

This commit makes the ChainService's IsCurrent use the block manager's
IsFullySynced instead of BlockHeadersSynced. This makes sure users of
the ChainService will wait until the filter headers are fully synced
befire starting using it.

lightninglabs/neutrino