title: List transaction timestamps (transaction_timestamp()) of all active (i.e. not yet committed) transactions
body: <p>I would like to query for the lowest transaction_timestamp() value among the currently active (not yet committed nor rolled back) transactions from all connections to my PostgreSQL database (I am not interested in just the transaction_timestamp() of my own session). </p>
  
  <p>I can get the transaction xid-s for such by txid_current_snapshot() and related, but I would like to find out what the transaction_timestamp for them are (or at least the lowest of the timestamps).</p>