[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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if repo.system_source.dbdriver == 'postgres':

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    sql('ALTER TABLE appears ADD COLUMN weight float')

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    sql('UPDATE appears SET weight=1.0 ')