[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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import ConfigParser

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try:

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    undo_actions = config.cfgfile_parser.get('MAIN', 'undo-support', False)

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except ConfigParser.NoOptionError:

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    pass # this conf. file was probably already migrated

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else:

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    config.global_set_option('undo-enabled', bool(undo_actions))