[FIX] openerp.api.Environment: move recomputation todos into a shared object

This fixes a bug which is usually triggered in module account_followup, but
does not occur deterministically.  Some recomputations of computed fields are
apparently missing.  Environment objects containing recomputations todos and
kept alive by a WeakSet, are removed by the Python garbage collector before
recomputation takes place.  We fix the bug by moving the recomputation todos in
a non-weakref'ed object.