[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.

import ConfigParser
try:
    undo_actions = config.cfgfile_parser.get('MAIN', 'undo-support', False)
except ConfigParser.NoOptionError:
    pass # this conf. file was probably already migrated
else:
    config.global_set_option('undo-enabled', bool(undo_actions))