Implement Continuous Queries

Users should be able to specify queries that run constantly on the cluster that feed into another time series. Here are a few examples:

```sql
select percentile(value, 95) 
from /stats\.*/
group by time(5m) 
into :series_name.percentiles.5m.95

select count(type) 
from events 
group by time(10m), type 
into events.count_per_type.10m
```

There should be a REST endpoint that returns which continuous queries are running on the cluster. There should be a DELETE endpoint to terminate a continuous query.