RFR: 8142366: Add develop_debug and develop_trace levels to Unified Logging

[Rachel Protacio]

Hello,

Please see my small changeset to add two develop levels to UL.

Summary: This adds develop (that is, non-product) logging levels to the 
Unified Logging framework in order to support performance, footprint, 
and usefulness-of-output considerations while maintaining the ability 
for the user to specify levels of verbosity, i.e. default, "debug," and 
"trace" levels.
Open webrev: http://cr.openjdk.java.net/~rprotacio/8142366/
Bug: https://bugs.openjdk.java.net/browse/JDK-8142366

I tested the added levels locally with sample log messages to ensure 
proper functioning. When I convert future tags to logging with these 
levels, those tags will have their own tests and inherently exercise the 
added levels.

Thank you,
Rachel