FlumeEventQueue removes fileID from fileIDCounts before commits are completed. See
FLUME-1417 for background. This series of events would be problematic:
File ( i )
n takes (file i is no longer in flume event queue fileIDCounts)
file roll -> current active file in directory (File i+1)
background worker kicks in -> removes file i.
Note that the commit for the n takes has not taken place.
We do keep 2 files per directory, but if a commit/rollback does not come and 2 files are rolled, we might have some issues.