This more accurately shows whether or not a query has been killed.
Instead of automatically removing it from the query table when it's
killed even though goroutines and iterators may still be open, it now
marks the process as killed. This should allow us to more accurately
determine if a query has been stalled and is still using resources on
the server.
This is related to #8848, but not directly connected.
This change provides a clear separation between the query engine
mechanics and the query language so that the language can be parsed and
dealt with separate from the query engine itself.
Jonathan A. Sternberg