#214 Need ability to tell a service to cancel one of its requests

[Chris Alkov]

It would be nice if the service interface provided a
means to notify a service that one of it's requests needs
to be cancelled. The service would then be responsible
for taking any cleanup, process termination, etc. steps
to properly cancel the request.

This would be particularly useful for services which have
long running commands. For example, services which are
used to install other products as they may have long file
copies plus long process executions for the actual install.