BulkInsert don't work for lazy-evaluated IEnumerable

[KurinAlex]

When using BulkInsert on lazy-evaluated Enumerable (for example created using yield) no records are actually inserted in the database.
According to source code it happens due to enumeration calls to entities before insertion (for example, calling entities.Count()). These calls leads to early enumeration of IEnumerable and it is empty by the time of insertion.

[borisdj]

Can you make a test for this, and do you have suggestion for fix.
One thing that comes to mind is to have a config for this so that is can be skipped.
ActivitySources was added with this PR: #457