Lucene tests sometimes stall until they timeout

[ScottDugas]

Specifically some people have noticed that LuceneIndexTest.fullDeleteHelper times out after 5 minutes rather than completing, but it's normal runtime is less than 500ms.

Similarly the newly added LuceneIndexMaintenanceTest.concurrentStoreTest can experience a similar issue.

[ScottDugas]

I believe this is caused by apple/foundationdb#11500 although I'm having trouble creating a valuable reproduction that doesn't involve all of record layer.