[PREVIEW] Avoid memory corruption and segfaults during ufs cache store maintenance

Hello,

    The attached patch avoids memory corruption and segfaults during ufs
cache store maintenance by removing being-deleted StoreEntry from ufs
cache replacement policies.

SMP Store changes added a SwapDir::disconnect() method that notified
entry's cache_dir that its entry is being destroyed. I added that code
so that shared cache_dirs can update their index when idle store_table
entries are destroyed on cleanup (completely avoiding idle entries
proved to be too difficult with store_table still around).

However, I did not realize that non-shared cache_dirs have the same
problem: Their StoreEntries may be deleted without cache_dirs knowing
that. UFS cache_dir now implement the SwapDir::disconnect() API to stay
in sync. It is likely that the problem was introduced by SMP Store
changes, but I do not know whether there is a better fix for it.

I also added assertions to catch cases where a dirty StoreEntry or
MemObject (i.e., an entry or object that is still in some replacement
policy index) is being destroyed.

TODO: This patch breaks "make check". We need more STUBs somewhere!

Alex.