Optimization for batch scheduler.

Getting the next batch queue entry to process was slow with a large database
due to the getFromQueue select query. This query is now replaced by one that
can make use of an index on the BatchQueue table.


git-svn-id: https://humgenprojects.lumc.nl/svn/mutalyzer/branches/refactor-mutalyzer-branch@296 eb6bd6ab-9ccd-42b9-aceb-e2899b4a52f1