[mc] In MC_ignore, keep the zeroed data out of the main snapshot

The previous implementation was zeroing out the snapshot data

   * so that it will be automatically ignored by the state comparator;

   * but not for the first snapshot in order to be able to backtrack
     to the initial state;

In preparation for the per-page snapshoting code, we

  1. zero out on the main memory;
  2. keep the zero-ed bytes out of the main snapshot regions;
  3. restore the zero-ed bytes.

This way the state comparison code and the per-page snapshoting code
do not have to worry about this. Moreover by normalizing, the
"ignored" bytes, we increase the possibility of sharing memory pages
in per-page snapshots.