New splitfile fetcher code (transient only)

This branch implements the new splitfile fetcher code, with some restrictions. The old code is still present. The new code is used for non-persistent non-cross-segment requests (i.e. <80MB, fproxy etc), with temporary files encryption turned off (i.e. physical seclevel LOW). It's configurable, turned off if memory is less than 256MB (partly because the old FEC and new FEC both use memory), and also turned off for auto-update (for robustness, though I have had this apparently working). There are unit tests for much of the new code, and I've done a reasonable amount of in-vivo testing, including on older content. AFAICS it works, but wider testing would be good. Further changes will not break anything because this is only for transient requests. Review requested, but IMHO it could probably be safely deployed to the network at large, and would probably be a performance gain. I will work on another branch for persistence support.