buf_pool->LRU_old_len.  However, we forgot to check if buf_pool->LRU_old
happens to point to b's successor in the LRU list.  If it does, we must
assign buf_pool->LRU_old = b.  The following invariants hold:

In the LRU list, the "old" flag should grow monotonically, i.e., it is 0
for the first few items and 1 from thereafter.

If buf_pool->LRU_old != NULL, it must point to the first item with old=1
in the LRU list, and there must be buf_pool->LRU_old_len old items in the list.

This should fix Mantis issue#50 and issue#68.