Add safe add_check_constraint

[ThHareau]

Adding a check constraint requires an ACCESS EXCLUSIVE lock, which prevent writing and reading in the tables as soon as the lock is requested.

Adding the constraint itself is rather fast, the major part of the time is spent on validating this constraint.
Thus safe-pg-migrations ensures that adding a constraints holds blocking locks for the least amount of time by
splitting the constraint addition in two steps:

1. adding the constraint without validation, will not validate existing rows;
2. validating the constraint, will validate existing rows in the table, without blocking read or write on the table