docs/config: add documentation for queue setting

Related to #305.
queue: rename queue

We use beanstalkd, not beanstalk :-)

Closes #305.
provision/juju: increase timeout in test

I don't care anymore :-)
app: fix data races

Just fixing the build, I didn't like these mutexes contention. I will
improve this later.
testing: use atomic operations in fakeHandler
queue: fix data race in test
testing: use a mutex to protect access in FakeQFactory
provision/juju: fix data races

Same motivation as f506e3fc7bea524ffd81d0b4101db640f6c5e4ad.