Switch from StringBuffer to StringBuilder.

Happy now, @cgray?
Collapse two catch blocks into one. Learn something new every day.
Small exception description tweak.
Add a new proxying resource to help test the new Apache HttpClient instrumentation in the agent.

Also stopped registering the InMemorySpanSinkSource since the UI is no longer hosted by the example service.
Adds support for getting the trace ID and span ID of the current thread as well as the ability to start a span with a given trace ID and parent span ID instead of relying on what's currently in the ThreadLocal storage.
Instrumentation support for Apache HttpClient requests including linking trace IDs and parent span IDs across HTTP requests.