obsdiscovery: extract push-obshashrange discovery in a function This will help us to simplify the code. The code is updated to directly use 'outgoing' missing so that is no longer needs to access 'futureheads'.

obsdiscovery: merge two distinct if statement It did not made any sense for them to be split.

obsdiscovery: extract '_canobshashtree' having this as a simple function will help use to generalise the obsdiscovery algorithm we use.

obsdiscovery: move code inserting discovery at the end of the file This makes it easier to reuser function defined earlier in the file.