[mc] Compute a single hash (64 bits) of the current state

[mc] Compute a single hash (64 bits) of the current state

This is an attempt to speedup state comparison by using
a very fast first pass.

Compared to previous attempt:

* use a simple 64 bits djb2 hash instead of SHA-1;
* add has much info as possible into the hash;
* do not add anything suspicious;
* it could be used for efficient state comparison
  * index for hashtable,
  * cache-friendly data-structure.