Batch totals incorrect

[donaldgray]

There are instances where items are being double counted for batches, meaning Batch.Completed + Batch.Error > Batch.Count. This was a previous issue seen, which #506 attempted to fix.

This has only been seen when doing large batch ingests, when thousands of images are being ingested and multiple instances of Engine are running which leads me to believe it's related to multiple different instances of the Engine incrementing counters at the same time.

Note that #491 may mean that this is unnecessary.