As developers must choose a content topic for their app.
The`WakuMessage` APIs have been changed to move `contentTopic` out of
the optional parameters. Recommendations for content topic can be found
at https://rfc.vac.dev/spec/23/.
Note that we currently only support one, and only one, pubsub topic for
a given instance across the codebase. The PubSub topic needs to be set
when instantiating the Waku* classes.
At this stage, we believe that most DApp will use, and only use, the
default PubSub topic. Some application want to use an alternative topic
but not use the default one so this behaviour should be fine. See #174
for details.
