Allow rollback to a given savepoint multiple times.

transaction/_transaction.py
    _remove_and_invalidate_after():  Don't remove the
    argument savepoint from the savepoint stack anymore.

    abort():  abort(True) relied on the subtxn savepoint
    being invalidated as a side effect of rolling back
    to it.  Invalidate it explicitly now.

    Savepoint.rollback():  Don't invalidate self.

ZODB/Connection.py
    Connection._rollback():  Pass a temp container to
    _cache.invalidate(), because that method clears the
    container as a side effect.  Clearing the index
    in a savepoint was a disaster if the savepoint was
    used again.

    TmpStore.reset():  Make a copy of the argument index.
    An alternative would be to make all callers make
    copies, but the only caller now thinks of its `state`
    argument as being opaque.

transaction/savepoint.txt
ZODB/tests/{testSubTransaction.py, testConnectionSavepoint.txt}
    Test the new semantics, and beef up the tests.

transaction/tests/savepointsample.py
    _rollback_savepoint():  Needed to copy the data.