Docker load test

These commits correct an unwanted behaviour of the injector: once a db adapter saveTask operation failed it no longer disconnects from the gearman job server.

Load testing is done by submitting 10 delayed tasks in parallel through the gearman job server and waiting for their execution. There are two instances of each component in the entire stack except of clients of which there is one.

Currently the test passes, but logs many 'Slave not updated' messages from the injectors' saveTask db query.