Add support for Redis logical databases

[scott113341]

I'm not sure what all is entailed here; I'm not familiar with Redis logical databases. We might already support this, or we might have to add more CLI options.

Without knowing much, it seems potentially useful to be able to sample all logical databases:

redis-keyspace-stats --url $REDIS_URL --all-dbs

There's probably a use case to sample a single logical database:

redis-keyspace-stats --url $REDIS_URL --db=2

Maybe there is also a use case to sample a subset of logical databases:

redis-keyspace-stats --url $REDIS_URL --db=0,3,5,19

More investigation needed!