Substate scanning can be made O(log n) rather than O(n)

[CjS77]

Currently, get_latest_substate_from_committee does a linear search for the latest version of a substate. This is O(n).

You won't notice for version < 20 or so, but once a contract is used for a while, and versions reach 100+ and/or are very busy (with lots of updates between caches), this will become a noticeable performance drag.

Since substate versions are all down, then one up, then all non-existent, a binary search can make this an O(log_2 n) search by:

1. Set last known down version to last_down_version.
2. Set version += step = INITIAL_STEP = 8
3. If state is still down, set last_down_version to version. Double step and goto 2.
4. If state is UP, we're done.
5. If state is down, update last_down_version <-- version. Set version = pvs_version-1.
6. Set pvs_version <= version.
7. Else state must be DoesNotExist. Set version = (version + last_down_version)/2
8. Goto 4.