Low Description
A streamOutSession acquire a reference on the sstable it will stream and release them as soon as each sstable has been fully streamed. However, since a stream session has currently no means to know when it failed, we'll keep references indefinitely (meaning until next restart) if their is a failure. One way a stream session could very easily fail is if the remote end dies. We must make sure we correctly release sstable references when that happens.
Note that it won't be bulletproof, there is probably other means by which a streaming could fail: a bug in the code throwing an exception, no space left on the receiving end, etc... But those are unlikely enough that I propose to care only for the case of a node dying for now and leave the bullet-proofing to CASSANDRA-3112.