Minor logging tweaks

[cfredri4]

This change makes UnicastHeader/3 toString() consistent with most other headers by wrapping in "[]" And changes two logs for new UNICAST4/NAKACK4 to debug to not be logged by default

[belaban]

I don't like the [ and ] braces around the headers, as a matter of fact I removed them from NakackHeader. If we want braces, this should be done uniformly in Headers.printHeaders

[belaban]

I added Headers.delimiters(String start, String end): you could use Headers.delimiters("[", "]") to use angle brackets for example.