API function for stopping mpolice and taking a
delta snapshot, i.e..
all mpolice owned memory chunks allocated since
id: a unique positive value for identifying this
a pointer to a constant array of struct
the last item is empty and has addr==NULL,
pbtrace==NULL, and size==0
error return: NULL (no current start, or
current start not for id)
the memory returned is owned by mpolice, do not try to write or
the memory returned will be reused on next successful stop call, do
to that memory around for longer than that.