Fixes a bug where the fact that an SR master leaves the primary view
gets missed. When two consecutive primary views have the same
membership we now assume that every SR needs to be rolled back, as the
system may have been through a state of only non-primary components.
Use iterators for scanning members vector in order to avoid
issues with integer signedness and range checks. The vector is
usually rather small and not in hot codepath, so performance
is here not an issue.
Added unit test for member_index() method.
