Ensure that we correctly expire (and cleanup) redis members

When a member has not heartbeated in a given amount of time
it should no longer be present in any of its associated groups,
to accomplish this we need to have a heartbeat key that each
member uses and have the group membership code fetch, check,
and clean members (if they are no longer alive).

Fixes bug 1386684

Change-Id: I89a8343ea9317c0522a6132ea065905b16b27b0d
Merge "Ensure that we correctly expire (and cleanup) redis members"