[migractions] Better handle removal of RQLConstraint in sync_schema
In rdef synchronisation, filter unmodified constraints before processing the
sets of old and new constraints.
Add a new `constraint_by_eid` method on RelationDefinitionSchema, which
eliminates the ambiguity of `constraint_by_type` usage when several
constraints of the same type are involved in the same migration.
Closes #3633644.
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