Fix getOldestStartTime

Summary: There was a tiny possibility that apc entries would be
freed immediately, even though they were still in use by a request, because
getOldestStartTime could return a value 1 greater than it should have done.

Reviewed By: @alexmalyshev, @jasone

Differential Revision: D1679072

Signature: t1:1679072:1415898450:3de211d0b25623b01c3018fe99fb0a9ae6be2fe5