[multi-sources-removal] Simplify ConnectionsSet internal structures and public methods since it now handles a connection to the system source only Related to #2919300 [jcr: adjust 3.17.11 migration, fix a number of bugs in new ConnectionsSet implementation, fix source.{open,close}_source_connections]

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for table, column in [

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        ('transactions', 'tx_time'),

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        ('tx_entity_actions', 'tx_uuid'),

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        ('tx_relation_actions', 'tx_uuid')]:

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    repo.system_source.create_index(session, table, column)

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commit()