3966 Remove clone from Membership and wrap it in Arc<RwLock>>

[pls148]

Closes #3966

This PR:

Converts internal storage of Membership object from a copied-around entity to a singular object wrapped in RwLock.

This PR does not:

Key places to review:

Testing code. It got a little tricky to make sure that each Membership object for each node being tested was wrapped in its own RwLock rather than having all of the nodes share the same Membership object

[lukaszrzasik]

I think the change is tricky as we now have a shared instance accessed in many different places. I have some comments. These might be important. Let me know what you think.

[lukaszrzasik]

I think it's good to go.