BUG-2459: do not convert NetconfMessage

Router's onNetconfMessage shows up as a hotspot in performance traces,
which turns out to be another case of coversion of XML document to a
string just for debugging/error handling purposes.

In case of a debug + error we are taking the hit of one toString()
operation, but that should be fine. We make that up by being smarter
about creating maps in the error path.

Change-Id: I78ec6ad44c4689b67bbc51184465c5092b0d8657
Signed-off-by: Robert Varga <rovarga@cisco.com>