routing/chainview: optimize neutrino implementation of FilterBlock
routing/chainview: optimize neutrino implementation of FilterBlock

This commit optimizes the neutrino implementation of FilterBlock method
of the ChainView interface. The old implementation would _always_ fetch
the entire block and manually scan through it. Instead, we can just
fetch the filter, and then if the items match, fetch the block itself.
This will save bandwidth during a lnd node’s pruning of the channel
graph after a period of dormancy.

LightningNetwork/lnd,ddustin/lnd,LightningNetwork/lnd,AndrewSamokhvalov/lnd,aakselrod/lnd,aakselrod/lnd,ddustin/lnd,AndrewSamokhvalov/lnd