BUG-770: NumberFormatException for datapathId

- changed long to bigInteger in order to avoid negative Long
- added unit tests for touched util-methods

(cherry picked from commit e7971f6ae3cfc260e14cd26df98d6eeed30665f8)
Signed-off-by: Michal Rehak <mirehak@cisco.com>
Signed-off-by: Michal Rehak <mirehak@cisco.com>
Change-Id: I71b2695b18917645eb58a6535e7b37196415dd1b