Sync core retains the local query result seq head

PuppetDB should synchronize with another instance more efficiently now. Previously it would synchronize each entity (factsets, reports, etc.) incrementally, holding open PostgreSQL queries/transactions throughout the entire process, which could take a long time. Those transactions could substantially harm database performance, increase table fragmentation, and if entangled with something like pglogical, increase transient storage requirements (by blocking WAL log reclaimaton). Now instead, the queries should completed up-front, as quickly as possible.