Add support for partitioned queries

[flimzy]

CouchDB 3.0.0 adds partitioning support, and doing a partition-specific query requires a different endpoint. https://docs.couchdb.org/en/stable/api/partitioned-dbs.html#db-partition-partition

[flimzy]

I think the least intrusive way to handle this will be to add Kivik support for a _partition option. i.e.:

db.AllDocs(ctx, kivik.Options("_partition": "foo"))

Then add a new PartitionInfo or PartitionStats method to handle the the _partition/{id} endpoint (https://docs.couchdb.org/en/stable/api/partitioned-dbs.html#db-partition-partition)