Before these changes, shutting down a node could be prolonged because of
mapreduce_service. `mapreduce_service::stop()` uninitializes messaging
service, which includes waiting for all ongoing RPC handlers. We already
had a mechanism for cancelling local mapreduce tasks, but we were missing
one for cancelling external queries.
In this commit, we modify the signature of the request so it supports
cancelling via an abort source. We also provide a reproducer test
for the problem.
Fixesscylladb/scylladb#22337Closesscylladb/scylladb#22651
cd50152522