Refactor ShardSpace to RetentionPolicy

* Remove regex since they will be written to or queried explicitly
* Rename retention to duration
* Remove duration
Add set default retention policy
Minor edits to replace shard space with retention
Fix impl and test for setting default retention policy
Update handler with retention policy stuff
Merge pull request #1141 from influxdb/refactor_shard_space

Refactor ShardSpace to RetentionPolicy