update_members should only accept a list of maps

[tsloughter]

The default peer service manager currently supports the nodes list as both a list of atoms and a list of maps. This results in some confusing hacks and logic to handle.

Requiring the list to be the map representation of a node would clear it up and allow to simply use sets to find the difference and intersection of the current membership and the updated membership.

[cmeiklejohn]

I will take care of this soon.