Improve snapshot performance for big traces

[Boyan-MILANOV]

Currently snapshots record every single memory write event to be able to restore a past state. This won't scale so well on very long traces that are very memory intensive. An alternative would be to snapshot memory on a per-page basis: save the whole page when it gets written for the first time, and don't record subsequent operations affecting this page.

• The cost for small traces is increased
• The amount of RAM needed to take snapshots on long traces could be reduced

We should probably allow both memory snapshoting strategies to be selected as a setting for advanced users.