Remove unnecessary extra batches in SaveChanges

[AndriySvyryd]

• Add API to signal a "batch wall" past which the commands can't be reordered. This would replace the current ModificationCommandBatch.Complete() method.
• When performing the topological sort only consider the FKs where both entity types have associated commands
• Consider grouping predecessor map by FK
• When using client-side value generation, we can stop doing multiple roundtrips when related entities are involved, since there's no need to fetch the primary key from the DB before sending the dependent. This could be a valuable optimization for people using client-side GUID generation (and hi-lo).

[AndriySvyryd]

Related: #3954