Stricter Conflict Detection modes

[timsehn]

A customer uses custom merge logic because Dolt's conflict detection logic is too lenient. Dolt does it's best to merge as much as it can conflict-free. But this strategy is not apprproate for all use cases.

We should have different conflict detection modes.

1. Schema/Data - Dolt will only merge data changes or schema changes, not both. If a schema change and a data change are made to the same table, a conflict is thrown.
2. No schema - Any modifications to schema on the same table are treated as conflicts.
3. No data - Any modifications to data on the same table are conflicts.
4. No row merge - Any modifications to the same primary key are treated as conflicts even if the cells that were updated in the row are different.

I'm sure we can think of more.

Potentially this should be controlled by a system variable that can be set globally or within a session.