Add lazy persistence back to KVStore::delete

[TheBlueMatt]

This is from discord:

• Should all deletes be performed in a lazy fashion now? I noticed the lazy flag was removed in Remove lazy flag from {KVStore,KVStoreSync}::remove #4116. I was surprised it was described as having no utility, as we ran into issues where a large channel monitor update consolidation (e.g. with maximumpendingupdates = 1000) would block other tasks from continuing if the 1000 deletes were performed sequentially in a non-lazy fashion. This might apply most to the sync fn use-case though.