1.0.15.31: thread-safe FIND-CLASS -- really this time

1.0.15.31: thread-safe FIND-CLASS -- really this time

  Call It Myopia: it turns out FIND-CLASSOID &co underneath FIND-CLASS
  (when called for non-existent classes) were not thread-safe either.

  * Get rid of *FIND-CLASS* hash-table, moving the actual PCL classes into
    corresponding CLASSOID-CELL (new slot PCL-CLASS).

  * Move classoid-cells from the infodb into into *CLASSOID-CELLS*
    hash-table. We want to be able to lock around

      (or (get-cell) (setf (get-cell) (make-cell)))

    and infodb isn't really designed for that. This is the crux of
    the breakage:

     *** parallel writes to infodb are not thread safe! ***

  * Lock over *CLASSOID-CELLS* and *FORWARD-REFERENCED-LAYOUTS*. The
    latter should not be really necessary as long as we don't
    assume (SETF FIND-CLASS) to be thread-safe, but easier to reason
    about it this way. ...and it would be nice for the SETF to be safe
    as well.

  Related work:

  * Don't create cells for non-exitent classes unless we know we are
    going to need them -- previously both FIND-CLASSOID and FIND-CLASS
    created a cell for every name they were called with, which is
    isn't too good. This is especially important as once created these
    cells never go away!