[Unison-hackers] Experimental "faster check" mode

[Jerome Vouillon]

This is great!

A few remarks:
- *Don't* store pseudo-fingerprints in the fingerprint cache.  The
  cache contents should reflect what is known about file contents, and
  thus not depend on Unison settings. Pseudo-fingerprints should be
  handled at a higher level.  In particular, the fingerprint cache
  provides a way to rebuild an archive rapidly, without having to
  rescan all files.  Your code is breaking that.
- You should check that the Xferhint code still works properly.
  I suspect we don't want to store pseudo-fingerprints there either...
- If I read the code correctly, when "fastercheckUNSAFE" is set to
  true, any new file will fail to transfer.  This is the safe thing to
  do (though the failure happen at a very late stage, once the file
  contents has been transferred) but is very restrictive.  I explained
  in my previous mail how we can perform a transfer safely in this
  case too, but a protocol change is needed.  (That would be useful
  when one replica is initially empty: Unison would be able to start
  transferring files right away.)

-- Jerome