[FIX] removed memory leak in SharedProjectDecorator

It might be not so *clever* to:

1. add object to a list instead of a set
2. never clear those collection

With every refresh of the labels the collection will grow and grow
until the heap space is exhausted.

Also fixed concurrency issues.

Change-Id: Id16a18db17d3786952496fd0aea305c0ae045e4a
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1118
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>