Fix up the propagation of X-Trace metadata from NodeManager to Containers (specifically, YarnChild and MRAppMaster containers
Let the examples in the examples jar start X-Trace tasks, useful for debugging and verifying that everything is working
Base64 encoding isn't quite compatible with HTTP headers because it uses a disallowed character '=' for padding.  For now just switch to Base16, which is less efficient but only includes alphanumeric characters.  Could tweak X-Trace base64 to not include padding but don't want to risk violating any existing instrumentation, no time to fix it