L::ProcessablePool: Remove redundant member variables

Remove several members that were not properly maintained in all cases.
Their purpose could trivially be re-delegated to methods in
L::ProcessableGroup.

Signed-off-by: Niels Thykier <niels@thykier.net>
L::ProcessablePool: Add .changes when merging groups

When merging a group (from a .changes file) into an existing group,
also add the new .changes file if possible.

Signed-off-by: Niels Thykier <niels@thykier.net>