BUG/MEDIUM: memory: Add a rwlock before freeing memory.

BUG/MEDIUM: memory: Add a rwlock before freeing memory.

When using lockless pools, add a new rwlock, flush_pool. read-lock it when
getting memory from the pool, so that concurrenct access are still
authorized, but write-lock it when we're about to free memory, in
pool_flush() and pool_gc().
The problem is, when removing an item from the pool, we unreference it
to get the next one, however, that pointer may have been free'd in the
meanwhile, and that could provoke a crash if the pointer has been unmapped.
It should be OK to use a rwlock, as normal operations will still be able
to access the pool concurrently, and calls to pool_flush() and pool_gc()
should be pretty rare.

This should be backported to 2.1, 2.0 and 1.9.