close SR transacions on equal consecutive views

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.

include/wsrep/view.hpp

@@ -103,6 +103,11 @@ namespace wsrep
         ssize_t own_index() const
         { return own_index_; }
 
+        /**
+         * Return true if the two views have the same membership
+         */
+        bool equal_membership(const wsrep::view& other) const;
+
         int protocol_version() const
         { return protocol_version_; }
         const std::vector<member>& members() const { return members_; }