TopicManager is not thread-safe when advertising topics

[scpeters]

I'm not able to share a backtrace here, but I've seen a case in which gzserver crashes during initialization, failing the assert at TopicManager.hh:118 with a message of "FindPublication returned nullptr". The backtrace I saw was from the advertisement of ~/request in the Scene constructor, called by the initialization of a WideAngleCameraSensor. Looking at the code for TopicManager::UpdatePublications, TopicManager::FindPublication, and TopicManager::Advertise, I don't see protection for the TopicManager::advertisedTopics variable, so I think it is not thread-safe.

I'm guessing that multiple threads attempted to advertise ~/request at the same time. It should be possible to write a test that advertises new topics from multiple threads to try to reproduce this.