Extract Default source enforcement logic
Extract Default source enforcement logic

Enforcing the constraint that there be exactly one default source
whenever there are sources present isn't really a concern of the
traditional CRUD methods that are provided by SourcesStore. This makes
this separation a little more explicit and provides a place to glue some
documentation onto what would otherwise be a confusing bit of business
logic in the write path of SourcesStore.

mark-rushakoff/influxdb,influxdb/influxdb,nooproblem/influxdb,nooproblem/influxdb,li-ang/influxdb,influxdata/influxdb,mark-rushakoff/influxdb,mark-rushakoff/influxdb,nooproblem/influxdb,nooproblem/influxdb,mark-rushakoff/influxdb,influxdb/influxdb,mark-rushakoff/influxdb,influxdb/influxdb,li-ang/influxdb,influxdata/influxdb,mark-rushakoff/influxdb,influxdb/influxdb,influxdb/influxdb,influxdb/influxdb,nooproblem/influxdb