Bug#905859: async-http-client build depends on libnetty-3.9-java

[Emmanuel Bourg]

Control: block -1 908023
Control: tags -1 + pending

I've prepared the upgrade of async-http-client to the version 2.5.3
which uses Netty 4.1, but it requires a new dependency (see #908023).
The update is ready in the Salsa repository.