Add DirectoryListener

[dr0i]

See #702.

This PR works in principle.
Be aware that there is a bug regarding the WatchService that can result in a loss of awareness of created files, e.g. if these files are created too fast in a row, or e.g. by moving a whole directory with files in it to the directory which is listened to: that directory would be watched, but the files in there won't be recognized. So it's not as inotify in unix contexts - it only comes close to it.

You may want to test it like this:

1. ./gradlew assembleDist (in the root of this branch to build the runner)
2. cd ./metafacture-runner/build/distributions/
3. tar xfz metafacture-core-702-addDirectoryListener-SNAPSHOT-dist.tar.gz
4. create a FLUX:

echo "default infile = FLUX_DIR + "tmp";
infile|
listen-directory|
print
;"  > directoryListener.flux

5. mkdir tmp (creates the directory to listen to)
6. metafacture-core-702-addDirectoryListener-SNAPSHOT-dist/flux.sh directoryListener.flux to execute the FLUX
7. touch 1 2 3 4 5 6 7 tmp/ to create some files

You should see as output the names of the files with absolute path (which could be given in the FLUX to open-file) and some logs (which are not going to the piped flux-command (as e.g. open-file)) are printed to stdout.
(Interestingly , you see that even "touch"ing 7 files consecutively the WatchService is fast enough to observe the creation of these files.)

If you want the listener to go down: trigger it with the specially named file:
touch shutdownEtlNow

We may want to discuss if it's necessary to improve the behaviour by building some workarounds. One idea would be to just traverse the given directory every n-th second and notate the filenames, if any new appear, to a Map and push these down the pipe. This would guarantee to not miss one file (at the cost of not instantly getting the filename if one was created.)

[dr0i]

functional review: @TobiasNx (and maybe @fsteeg as this PR is supposed to be part of a workflow in the RPB context).
discussion (re bug and workaround): also @blackwinter
code review: @blackwinter (or @fsteeg )