Query Management Support

[jwilder]

This is a tracking issue for discussion related to query management. See #655 #654 #5838.

[jsternberg]

I've gotten support for displaying running queries with SHOW QUERIES and modified the query language to allow saying KILL QUERY <number>, but the second isn't fully implemented in a way that works well.

First, in the current query engine, the interrupt will only take place after the first row gets emitted (name/tag combination). It also will not interrupt in the middle of calculating a query. We might have to add interrupt points inside of the iterator to get this functionality to interrupt a count in the middle of calculation, but that may result in slower aggregations.

I'll have to benchmark some attempts to see what happens.

[jwilder]

Related to query limits: #6024