Queue drain enhancements, Repository safety.
* `MongoStorage` now properly throws a `ServiceException` when it's started without an underlying database
* Major `MongoStorage` refactoring featuring actual `MongoException` handling
* Failures to persist should no longer lead to multiple threads caught in infinite loops
* Activity suspension of draining threads when error rate gets too high
* Configurable behavior on above suspension

fixes #135
Merge branch 'master' of github.com:ever-been/everBeen
* Fix `ClusterContext` instantiation

Conflicts:
	been-api/src/main/java/cz/cuni/mff/d3s/been/api/BeenApiImpl.java