Re-enable multiple parallel DML operations for a table

Previously Columnstore can handle multiple concurrent DML queries but this functionality had been disabled b/c of inconsistent snapshots produced by the current versioning. Concurrent access mechanism had been replaced by a table-level lock taken by any DML.
It is necessary to estimate the effect of the lock that presumably decreases DML QPS and find the ways to replace the lock with more granular locking(dependency graph based).