for savepoint management are:

- All savepoints for a transaction should be invalidated when the
  transaction commits or aborts

- If a savepoint is rolled back, then all savepoints after it within 
  a transaction must be invalidated.

We previously implemented these requirements by organizing transaction
savepoints into a doubly linked list.  This was overkill.  We didn't
have need for such fine-grained ordering.  This strategy had the
disadvantage that it kept all savepoints around until the transaction
ended.  Savepoints could be expensive to keep and it's possible that
some applications could keep a lot of them.

The new stragey is to:

- Keep weak references to savepoints.  We can forget about savepoints
  that the application isn't using.  Any resources used by these
  savepoints can be freed.

(We have to keep a strong reference to the last savepoint used for
  a subtransaction.)

- We assign indexes to savepoints within a transaction.  When a
  savepoint is rolled back, in addition to invalidating that
  savepoint, we also invalidate savepoints with a higher index.

A side effect of this change is that code using the savepoint API
should interfere less with code using subtransactions.  Of course, we
really need to phase out code that uses subtransactions.

It is likely that we can leverage this change in strategy to speed
creation of ZODB connection savepoints.  Creating a ZODB connection
savepoint now requires copying the savepoint storage index.  This
index could become large.  If applications aren't holding on to old
savepoints, then it is possible that we could avoid this copy.