potential cbdata problem with multiple store dirs.

storeIOState is registered as CBData with *different* callbacks
depending on which storedir open routine is called first.

i.e.
(store_io_diskd.cc)
 CBDATA_INIT_TYPE_FREECB(storeIOState, storeDiskdIOFreeEntry);

I think this is potentially pretty bad... but I'll be addressing this in
unify-IO.

Do we need to do something for 2.5 ?

Rob