Clearing the stream-isolated LRU caches not possible

[JeremyRand]

In my current stream isolation PR, stream ID's and their associated LRU caches accumulate forever. The Tor stream isolation codebase will tell us when a new NYM_EPOCH has been reached; this is a safe point to delete all currently existing stream ID's and their LRU caches. (Of course, the new NYM_EPOCH event is not actually exposed via the Prop279 protocol, so we'll have to work with the Tor folks to get that added to the spec.)