Add quick id function to speed up import dedup

[Jason Gerard DeRose]

We need a quick dedup to run when importing files that may have already been imported. This is especially important when importing large files (say 4GB MOV) over slow USB2. A good algorithm would probably be something like:

  hash(utf-8 repr of mtime + 1024k + last 1024k)

Corner cases are when total file is less than 1MiB and when less than 2MiB.

This function should go in dmedialib/filestore.py and should have a signature like:

  quick_id(filename=None, fp=None)

When you can pass either a filename or an open file handle.