Promote SQL Server bulk insert logic to relational ReaderModificationCommandBatch

[roji]

SqlServerModificationCommandBatch has a bulk insert logic, where multiple insertions are coalesced into a single, efficient MERGE statement. We should promote the infrastructure for coalescing multiple modification commands into a single SQL statement to relational:

• It would allow other providers to more easily implement bulk insert etc.
• It's currently a bit tricky to implement the logic only in SqlServerModificationCommandBatch, without ReaderModificationCommandBatch knowing anything about it (e.g. this). Adding the proper hooks in ReaderModificationCommandBatch would simplify this.
• We should make this support non-insertion as well (e.g. Implement bulk deletion with SaveChanges #27550).