kvstore: begin rewrite (ErrNotFound ⇔ found bool). mv kvstore kvstore-1
kvstore: rewrite Store and TransStore without ErrNotFound
kvstore: rewrite memkv, Store&TransStore tests
kvstore: rewrite sqlite3kv
kvstore: rewrite postgresqlkv

And fix a recent test-rewrite: the rewrite wasn't correct
kvstore: rewrite mysqlkv
kvstore: rewrite prefix stores
kvstore: rewrite KeyCollection, KeyQueue and their Iterators
minor: kvstore: go fmt
kvstore: end rewrite; rm kvstore-v1/