* Hundreds have been killed in "Middle Belt" violence
* Attacks not linked to Boko Haram insurgency
* Land disputes underlie ethnic and sectarian tensions (Adds witness accounts)
By Isaac Abrak
KADUNA, Nigeria, March 16 (Reuters) - Gunmen shot, hacked and burned to death at least 100 people and razed homes in central Nigeria, a region riven by disputes over land, religion and ethnicity,...