[INTERNAL] addressed FIXME in ContributionAnnotationManager

Removing elements inside of an iterator loop but not using the iterator
for removal may has surprising side-effects.

The returned null value from model.getPosition was due the fact that a
history cleanup was triggered which removed additional annotations which
were still returned by the iterator.

Change-Id: Iaf3a3b985b5bd1c6bdc1d24ad75f6c3a8dab77e9
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1784
Tested-by: Jenkins CI
Reviewed-by: Damla Durmaz <ddurmaz88@gmail.com>
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>