New() launches background calculateSizeAndPopulateIndex which scans
disk and calls insertBatch. Create does not wait on initDone, so a
file can be written and indexed, discovered by the scan, then removed
from d.info and disk by EvictLRU/EvictBySize. insertBatch then
re-inserted the stale discoveredFile without checking the path still
existed, so Stat succeeded from the index while os.Stat failed.
Re-stat under the lock and skip gone files so evicted keys are not
resurrected.
Fixes#18.
s1d3sw1ped_bot