Implement a TFile cache for RootEmbeddedFileSequence

[dan131riley]

In replay mode, RootEmbeddedFileSequence tends to do a lot of TFile thrashing opening PU files because thread assignments don't match, so a sequence that followed one thread/TFile in the previous step gets randomly assigned to threads in the following step. Caching the LFN-hash to LFN mapping would help some, caching the LFN-hash to TFile* would help a lot more.

I commented on this when I made the MixingModule threaded, but at the time it didn't seem like a problem. With more threads it looks like it is becoming more of an issue, and could be high impact for some specific WFs, especially HI production.

[dan131riley]

This is related to, but not dependent on #126 (and should be much simpler than the general TTreeCache usage)