ISPN-4733 Investigate potential issues with System.nanoTime() overflow

* Fixed TestingUtil calculation for when to stop trying
* Changed stats to not error if nanotime is -1 (extreme edge case)
* Also fixed TransactionTable when clearing old txs
ISPN-3938 AdvancedAsyncCacheLoader.process() concurrency issues

Simplify ExecutorAllCompletionService to track the tasks itself instead of
using an ExecutorCompletionService.