Make version counter in mlist_t volatile

The log manager is the only one that uses the mlist_t versioned list. The
counter that keeps track of the version number was not modified using
atomic operations meaning that the compiler is free to optimize away parts
of the lock-free versioning mechanism that uses it.

To prevent this optimization, the variable is declared volatile. A rewrite
is direly needed but it cannot be done in 2.2.

server/core/internal/mlist.h

@@ -32,7 +32,7 @@ typedef struct mlist_st
     bool           mlist_deleted;
     size_t         mlist_nodecount;
     size_t         mlist_nodecount_max; /**< size limit. 0 == no limit */
-    size_t         mlist_versno;
+    volatile size_t mlist_versno;
     bool           mlist_flat;
     mlist_node_t*  mlist_first;
     mlist_node_t*  mlist_last;