Add thread dump debug option

[SylvainJuge]

What does this PR do?

Adds an internal option that triggers a thread dump at regular interval.
minimal frequency is limited at 100ms

One of the expected usages is when the application code is not known and relies on unsupported technologies, in this case trace_methods has to be configured for transactions, the expected usage scenario is:

• configure the agent with log_level=debug and thread_dump_interval=1000ms
• start the application and trigger a few transactions manually
• analyze the agent logs with the thread dumps to find relevant classes and methods to instrument.
• thread dump interval needs to be adjusted relative to the usual transaction duration, for very fast transactions less than 100ms triggering more transactions will be required to get at least a few thread dumps that contain relevant application code.

Checklist

• This is something else
  • I have updated CHANGELOG.asciidoc

[JonasKunz]

Unfortunately this option seems less useful than expected: The ThreadInfo.toString() of the returned values from ThreadMxBean.dumpAllThreads only prints the top 8 stack frames for each. If we want this feature to be useful, I think we'd need to print the stacktraces ourselves.