[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.

# -*- coding: utf-8 -*-
# copyright %(year)s %(author)s, all rights reserved.
# contact %(author-web-site)s -- mailto:%(author-email)s
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"""%(distname)s precreate script, executed at instance creation time or when
the cube is added to an existing instance, before the schema is serialized.

This is typically to create groups referenced by the cube'schema.
"""