dolt verify-contraints does not check unique constraints

[timsehn]

Related to: #6319

ricardoreiter@MacBook-Pro-de-Ricardo bug % dolt sql -q "select * from tableA"
+-------+------+------+------+-------+
| keyC  | col1 | col2 | col3 | dataA |
+-------+------+------+------+-------+
| key-b | 1    | 2    | 3    | test1 |
| key-c | 2    | 2    | 3    | test3 |
| key-d | 2    | 2    | 3    | test4 |
+-------+------+------+------+-------+

ricardoreiter@MacBook-Pro-de-Ricardo bug % dolt sql -q "CALL DOLT_VERIFY_CONSTRAINTS()"
+------------+
| violations |
+------------+
| 0          |
+------------+

[fulghum]

I've got a prototype that reuses the existing merge logic and finds constraint violations from within dolt_verify_constraints(). It seems like this approach will work, but still needs lots more testing and figuring out a way to pull out the collected violation info from the merge.