Update dropwizard3 to use logback13, correct logback13 issues

[kanderson250]

Updates dropwizard3 to use the logback13 extension instead of logback. (logback did appear to be compatible with the newer versions of the module, but better to keep logback dependencies roughly aligned).

In the process, I found a couple things in the logback13 module that I fixed here as well to make it consistent with vanilla logback:

• The NewRelic: prefix should be stripped off our metadata log attributes before they're written to the generator
• Empty context values should not be added to the JSON layout
• The config to enable/disable mdc should be respected (current implementation always writes context attributes to the JSON layout for log events, even if -Dnewrelic.log_extensions.add_mdc=false).