Add ability to kill a running query

[pauldix]

#654 Will add functionality to see which queries are running in the cluster. We should have another query that will allow a user to kill a running query.

kill query 23 on hosta

To kill a query we'll need the host it's running on and the query id.

[tsenart]

What is the semantic difference between killing a query and deleting a query?
Through a brief code exploration I can see we already have functionality for the later.

// in coordinator/coordinator.go:97
if query.DropQuery != nil {
    if err := self.DeleteContinuousQuery(user, database, uint32(query.DropQuery.Id)); err != nil {
        return err
    }
    continue
}
// ...

[tsenart]

I also assume the inconsistence in naming between the query name drop and the correspondent action name delete is an artifact of technical debt.

[pauldix]

deletes are against continuous queries. This issue is to address any kind of regular query running across the cluster.

I don't think it's possible to complete this issue without completing #654 first.

[beckettsean]

Related to #411

[jwilder]

Fixed via #5950