Use parallel connections when snapshotting using Replication Slots

There is no need to use only one transaction to obtain a consistent view. We only need to keep the transaction that exports the snapshot open until other transactions are done with that same snapshot. So there's room for improvement where using multiple parallel connections can help snapshotting faster

See Snapshot Synchronization Functions at https://www.postgresql.org/docs/9.4/functions-admin.html#FUNCTIONS-SNAPSHOT-SYNCHRONIZATION