[TASK] ADWD-1532 Optimize Solr Queue

This resolves the stacking of requests to update the solr which would
be useless, since the whole dataset is exported on each change. So we
check if there is already something on the queue and only add new
requests if there is no request pending.
Merge pull request #190 from subugoe/ADWD-1532

[TASK] ADWD-1532 Optimize Solr Queue