Defines query processing of an asynchronous completion task.
The {@link #query(CompletionResultSet, Document, int)} abstract method
needs to be implemented to define the asynchronous querying behavior.
In addition filtering of the result set computed during querying
can be implemented by overriding the
{@link #canFilter(JTextComponent)} and {@link #filter(CompletionResultSet)}.
Check whether the task corresponding to this query was cancelled.
Subclasses should check this flag and stop the query computation.