publish messages from background threads

[ipa-jsk]

The usage of Time.realtimeSinceStartup in RosTopicState.cs allows publishing only from the main thread. Unity throws an error when trying to access Time.realtimeSinceStartup from other threads.

m_LastMessageReceivedRealtime = Time.realtimeSinceStartup;

Possible workarounds could be
02c37cb
m_LastMessageReceivedRealtime = ((float)DateTime.Now.Hour + ((float)DateTime.Now.Minute * 0.01f)); (works somehow, but actually this is not the realtime since startup)