Moves the drbbdup restore state event to a very early priority and has it modify the restore state parameters to focus on the containing copy of the block where the restore resides. It modifies the instruction list and the cache start pc.
Adds a new test drx-scattergather-bbdup by adding drbbdup to the existing drx-scattergather test. This test fails to restore without the fix described above, due to the scatter/gather expansion restore state machine getting confused across multiple copies.
Does not augment drbbdup's restore to handle the no-ilist case: leaves that for the general #3801 mechanism.
Issue: #5686