Cleanup message logging in netconf handlers

This wraps LOG.trace() invocations in LOG.isTraceEnabled(), as they
construct potentially big objects which may end up being unused.

Also cleans up method declarations to not use Exception, as Sonar warns
about these.

Change-Id: I8a1a01291b04eea0823b72e254f9ca4cc11557ea
Signed-off-by: Robert Varga <rovarga@cisco.com>
Implement NetconfMessage.toString()

Rather than explicitly converting the internal document intoa string
before logging it, teach NetconfMessage how to do toString(). This way
we do not have to check Logger configuration and pass NetconfMessages
directly. The logger will cann toString() as needed.

Change-Id: I07a18f32a1500784b81f52f0412acd73123a161e
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge changes I07a18f32,I8a1a0129

* changes:
  Implement NetconfMessage.toString()
  Cleanup message logging in netconf handlers