depth: update depth to code to reuse ancestors depth Computing the depth of all N revs is no longer 'O(N**2)'

obsdiscovery: extract a smarted depth in utility The function is reusing previous depth for ancestors unless this is a merge.

obsdiscovery: simply some of the missing computation

discovery: implement some range based discovery This use the previously implemented methods to implement some discovery and fetch method using ranges. There is currently major performance issue and some aspect of the protocol will changes.