Implement the C++ internals of the versioned storage library

[ivte-ms]

This should be a separate static library (consumed by the main StateSync library) with a very low-level interface, so that all low-level elements can be tested separately without exposing them through the main library's interface. Features that should go in first:

• Two-level ({key:sybkey}=>value) map-like versioned storage.
• Support for atomic transactions that validate and modify multiple keys and subkeys.
• Support for multiple immutable snapshots of different versions, that are not influenced by ongoing modifications.
• Deterministically ordered iteration over keys and subkeys within keys.

Future commits:

• Subscribing to changes (of individual keys and subkeys).
• Fast-forwarding the state of one instance of the storage to a newer state of another instance of storage (will be used by the StateSync directly).