Cleanup framing mechanisms

This gets rid of pre-instantiated NetconfMessageHeader in favor of
creating a static constructor method.

Furthermore allow our output chunking to be variable and bump the
default from 1K to 8K. The upper limit on chunk size is 16M.

Change-Id: I0ab4405fc7d750cca5875049f03b053aa80037ef
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "Cleanup framing mechanisms"