utxoscanner: adds batch rescan memory optimizations + reorg
utxoscanner: adds batch rescan memory optimizations + reorg

This commit extends simonhorlick's original work on batched
utxo rescans with the following items:
 - cache outpoint filter entries
 - batched discovery of initial txns
 - reuses backing array for filter entries
 - handles newfound blocks added during rescan

lightninglabs/neutrino