Create ModificationList for transactions

Create a modification list with the following requirements:

• Append only list
• Thread-safe
• Be able to snapshot (state transfer needs it)
• Be able to freeze (read-only when the prepare phase starts)
• Avoid unnecessary copies

Optionally:

• Rejects clear commands (assert)
• Handle local modifications (CACHE_MODE_LOCAL flag)

The main usage for this list is to append and iterate.