Leakage of references to SSTable on unsuccessful operations

This is a little bit tricky to describe correctly as I can talk about the symptoms only. I hit this issue when testing CASSANDRA-18781.

In a nutshell, when we go to bulkload an SSTable, it opens it in SSTableLoader. If bulkloading fails on server side and exception is propagated to the client, on releasing of references, it fails on this assert (1). This practically means that we are leaking resources as something still references that SSTable but it was not tidied up (on failure). On a happy path, it is all de-referenced correctly.

I think that this might have implications beyond SSTable loading, e.g. this could happen upon streaming too.

(1) https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/io/sstable/SSTableLoader.java#L245