the rules around acquiring locks have been strictly defined and
implemented, and general lock usage has been improved, to fix and
prevent several different issues.
a vm-region is now destroyed in two separate steps:
1. it is "killed": all mappings are unmapped and deleted, the
region is removed from its parent, and the region and all of
its sub-regions are marked as "dead", preventing any
further actions from being performed with the region.
2. it is "destroyed": the vm-region object is de-allocated when
the last reference/handle is closed. the references that this
region holds to any sub-regions are also released, meaning
these regions may also be de-allocated too.
