Only gc the oldest report/event partition in the normal case

PuppetDB will only drop the oldest report or events partition during the normal, periodic gc when there's more than one candidate. This is intended to decrease the length of time we might block pdb operations since with the current arrangement, the drop will attempt to acquire an exclusive lock on the entire table (i.e. reports, not just the partition), and so will block all subsequent access to that table, until it finishes.