make SSG groups mutable after creation

Currently, SSG group views cannot be modified after the group has been created, but this prevents us from adding functions for dynamically leaving/joining groups (or for modifying group views in the face of SWIM failures).

The necessary semantics for implementing dynamic group membership are easy enough to sort out, but the roadblock is finding a suitable data structure for storing group membership views. We are currently using an array structure to store group membership views which works great for static groups with member IDs corresponding to indices of the array (e.g., dense ranks). However, this solution is probably not the best for groups which have frequent membership changes, as it probably reallocating arrays frequently and possibly dealing with sparseness in these arrays as group members fail.