Broken Traces with JRuby and Tomcat


Here are the list of issues related to the broken traces:

- [x] __Init is sent during active trace context
- [x] Uninitialized constant AbstractMysqlAdapter (non-fatal cosmetic) #93
- [x] Don't initialize context or use X-Trace Req header if there is already one active
- [x] Don't insert X-Trace response header if we had an incoming context
- [x] Stack trace accessing sampleRateConfig source under tomcat
- [x] Validate no-op mode without the JOboe agent loaded
- [x] Validate Tomcat Rails to plain JRuby Rails cross app tracing (and vice versa)
- [x] Re-validate MRI Ruby cross app tracing
- [x] Update oboe-heroku gem to follow changes in this PR
- [x] Validate tracing on Heroku
- [x] Validate cross app tracing with java traceview api jar