add file size hash; make image acquisition date optional

Adds the `File-Size-64` hasher, adjusts `Adler-32` and `CRC-32` to use Guava implementations, and tweaks some import log retrieval.

To test,

* *without* this PR, try importing some images with various hashers set, including `Adler-32` and/or `CRC-32`
* upgrade to `OMERO5.1DEV__7`
* import some of the same images with the same hashers, and also `File-Size-64`
  * and check that you can view the import logs
* verify that
  * the same images with the same hasher have the same hash regardless of if they were imported with or without this PR
  * for files imported with `File-Size-64`, their hash is a small-endian representation of their size
* try setting image acquisition dates to `null` and check that this does not cause obvious problems: for instance, that images can still be viewed, exported, etc.

--rebased-from #2741 and fixes http://trac.openmicroscopy.org.uk/ome/ticket/12465
It seems a generally flaky test class so I went for minimal changes. I can push some cleanup that I hope improves it, but it won't be testable for now.