Make GC cancellable when called programmatically

Make GC cancellable when called programmatically

Sometimes, it is necessary to cancel a garbage collection operation.
When GC is called using the standalone executable, i.e., from a command
line, Control-Cing the process does the trick. When calling GC
programmatically, though, there is no mechanism to do it.

Add checks in the GC process so that a custom cancellable progress
monitor could be passed in order to cancel the operation at specific
points. In this case, the calling process set the cancel flag in the
progress monitor and the GC process will throw an exception that can
be caught and handled by the caller accordingly.

Change-Id: Ieaecf3dbdf244539ec734939c065735f6785aacf
Signed-off-by: Hector Caballero <hector.caballero@ericsson.com>