Replace redis-cli entirely with librdb

[gaurav]

We currently use redis-cli to pipe the RDB files into a Redis database, but that code is sadly out of date and only supports RDB versions up to Redis 6. So I've needed to replace it partially with librdb (PR #1044), but still needed to use redis-cli for the actual piping. With some recent changes to librdb (e.g. redis/librdb#87), it might be possible to use librdb entirely.