Based on Avi's feedback:
> We generally have a public abort() only if we depend on an external
> event (like data from a tcp socket) that we don't control. But here
> there are no such external events. So why have a public abort() at all?
If needed in the future, we can consider adding
get(abort_source&) to allow aborting get() via
an external event.
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>