Fs cleanups

A few minor things I found while testing open tickets (7856, 11262):
 * Remove unused mention of log4j's MDC
 * On import failure, raise an exception rather than permit a NPE. 
 * Enable more use of the server-side memoizer cache.

Of these, the last is the only that one that really requires re-testing. While importing SDT files, each fileset log had at least one instance like this:
```
/ManagedRepository/root_0/2013-12/05$ grep "no directory given" *.log
07-38-49.111.log:2013-12-05 07:38:52,827 DEBUG [                   loci.formats.Memoizer] (3-thread-3) skipping memo: no directory given
```
meaning that multiple calls to `SDTReader.setId` were being made. For any file format which takes a substantial amount of time on the `setId`, this could be substantial large savings.

/cc @melissalinkert, @pwalczysko, @jburel