Add ability to combine multiple structures and preserve snapshot consistency

[Erigara]

Right now there is a possibility that when combining multiple structs created read snapshot would be inconsistent.

Right not it's solved by adding single read write lock:

1. during commit write lock is held
2. during snapshot read lock is held

This insures that snapshot is not created in the middle of commit.

There might be alternative approach proposed here.
However it might require contributions to concread and path further is not yet fully clear.