Revise threading model of DOM

Currently DOM has multiple sync points based on mutexes and lot's of shared_ptr's (managed values of which are not thread-safe) stored in maps of DomEnvironment and DomUniverse.

Taking into account that DOM is indeed being used in the multi-threaded environments. It would be helpful to:

1. Figure out the usecases and thread management
2. Improve explicitness of the sync points
3. Improve thread-safety. There might be potential data-races / deadlocks, we're not aware of
4. Reconsider thread sync approach. maybe we can do everything in lock-free manner
5. smth else