Make constants really constant

Make constants really constant

This triggers a major sonar warning, and is insecure as these fields could
get modified by anyone, throwing the system out of whack.

Also optimize comparison and make sure the netconf QNameModule is cached.
Also prevent unneeded QName instantiation.
Also use an immutable list to hold arguments, preventing a copy in
builders.

Change-Id: I74647c444ec273066fc4727618b35c92386ba77c
Signed-off-by: Robert Varga <rovarga@cisco.com>
(cherry picked from commit fc48504605170d4d37346738602de388daa36f25)