Intermittent failures of HonoClientImplTest
Sometimes, the HonoClientImplTest fails with this error:

> Running org.eclipse.hono.client.impl.HonoClientImplTest
> 10:08:29.303 [main] DEBUG o.e.hono.client.impl.HonoClientImpl - creating new message sender for telemetry/tenant
> 10:08:29.313 [main] DEBUG o.e.hono.client.impl.HonoClientImpl - already trying to create a message sender for telemetry/tenant
> Tests run: 1, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 0.619 sec <<< FAILURE! - in org.eclipse.hono.client.impl.HonoClientImplTest
> testGetOrCreateTelemetrySenderFailsIfInvokedConcurrently(org.eclipse.hono.client.impl.HonoClientImplTest)  Time elapsed: 0.567 sec  <<< FAILURE!
> org.mockito.exceptions.verification.WantedButNotInvoked: 
> 
> Wanted but not invoked:
> protonConnection.createSender(
>     "telemetry/tenant"
> );
> -> at org.eclipse.hono.client.impl.HonoClientImplTest.testGetOrCreateTelemetrySenderFailsIfInvokedConcurrently(HonoClientImplTest.java:91)
> 
> However, there were other interactions with this mock:
> protonConnection.isDisconnected();
> -> at org.eclipse.hono.client.impl.HonoClientImpl.checkConnection(HonoClientImpl.java:361)
> 
> 
> 	at org.eclipse.hono.client.impl.HonoClientImplTest.testGetOrCreateTelemetrySenderFailsIfInvokedConcurrently(HonoClientImplTest.java:91)

See here for example:
https://travis-ci.org/eclipse/hono/builds/217209692