[WIP] Implement basic support for sharding the database.

This commit adds experimental support for a sharded mailsync database, provided
that each sync instance operates only on one shard. This allows you to
multiplex multiple sync engines and databases behind a single API server, for
example. Namespaces are managed via a master database, and namespace_ids are
unique across all shards. For details of the strategy, see inbox/sharding.py.