The Document Level Security BitSet cache stores a secondary "lookup
map" so that it can determine which cache entries to invalidate when
a Lucene index is closed (merged, etc).
There was a memory leak because this secondary map was not cleared
when entries were naturally evicted from the cache (due to size/ttl
limits).
This has been solved by adding a cache removal listener and processing
those removal events asyncronously.
Backport of: #50635