Cleanup cache.log output of protocol flow

[yadij]

In debug sections dedicated to displaying protocols, level-2 is reserved for logging a human readable trace of messages sent and received.

Provide a DBG_PROTOCOL constant easily used for these log outputs. This also provides an easier way to search for examples of the syntax use by these traces.

Also, update log output syntax for protocols which already contain message flow debugs().

Future work:

• display traces for more protocols
• Enhance debug_options to support tracing just the protocol(s) selected by their names.
• add traces to clarify protocol switching on a connection