Add Shard Spaces and Retention Policies

Fixes #571. Removed the old long term and short term shards. Now if the
user doesn't manually create a shard space, a catchall called "default"
will be created the first time they write data in.

Added API endpoints to list, create, and delete shard spaces. Made a
breaking change to the old get shards API by changing the format since
shortTerm and longTerm shards aren't guaranteed to be present.

Shard spaces can be mapped to a single database. Retention policies are
enforced per server by a background routine that periodically checks
shards in spaces that have retention policies to see if they have passed
their expiration.
Get rid of the shards slice
Return an error if there are no shards to query
Add command line option to load a database configuration.
use one field instead of two in the test