use new SDNV package version

* update Contact Header EID length field implementation
* update tests to work with new SDNV format
add header validation and verification

* define default protocol version as npm package version
* add helper to check if protocol is valid
* add helper to check if version is supported
change header write operation order

* write the header first
* validate the counterpart header next
rewrite ContactHeader constructor

* remove valueOf method
* define ContactHeader as Buffer by itself
* add accessors for the appropriate fields
* adapt ContactHeader exchange routines and tests to this new vision
header code refactor

* extract buffer wrapping process into a helper function
* change field accessors to return Buffers only
* remove useless helper functions
update some error messages