Document the effects of concurrent state transfer/rehashing

Rehashing/state transfer for joiners is sequential. The process should be detailed in a wiki page to help people understand this process.

From Mircea's original comment on this JIRA:

"How are the overlapping rehashing/state transfers managed. E.g. one node JVM crashes ->
rehashing/state transfers, the crashed node comes back to alive (e.g.
through automatic restart) before the rehashing/state transfer is
completed. What is the policy to manage this situation?

• add a UT to catch the behavior of the previous scenario
• add a wiki page to describe state transfer and how multiple nodes join etc"