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.

Primarily, _tindex should have been a dict all along, pack()
worked too hard at the end, and loadEx() is a minor variant
of load().

Ideally, load() should call loadEx() instead, but we really
don't want "an extra" Python-level call here (heavily used).

loadEx():  deleted pointless call of self._read_txn_header().

ServerStub.loadEx() comments:  these were obviously wrong in
several ways, but I don't know the full truth.  Better to
say so up front than to leave them clearly wrong, though.

The BTree minKey() and maxKey() methods gave a misleading message if no key
satisfying the constraints existed in a non-empty tree.

In some error cases when reading a FileStorage index, the
code referenced an undefined global.

Such code never made sense; it was all of the form

    t32 = 1L << 32
    if something < 0:
        something = t32 - something

which is a way to change negative integers into
gigantic positive integers greater than 4 billion, and
never made sense in context.  Possibly

        something = t32 + something

was intended, but that never made sense in context either.

In any case, `something` is invariably obtained from
struct.unpack using the "H" format code, and that never
returns a negative integer to begin with.

Clarified that the ``minKey()`` and ``maxKey()`` methods raise an exception
if no key exists satsifying the constraints.

Also improved the English in other interface docstrings.

Make undo{Log,Info} arguments act like Python slice indices when
both are non-negative.  The code used to do that before ZODB 3.4a9,
but changed to match ZODB's UML documentation.  Alas, some
(untested) code in Zope relied on the actual behavior (see the
collector report).  Changed code, docs, and tests to bless the
old behavior in these cases.

DemoStorage.UndoLog:  this was wrong in several ways.  I'm still
unsure about why it skips "packed" transactions.  That doesn't seem
right, but I don't have time to wonder about that now.

Windows trigger.__init__:  Simplify more.  The trigger can connect
after the other end has done  bind() and listen(); no need to wait
for an accept() too, and so no need to fiddle with blocking/
non-blocking or "expected" socket.error's either.

remove_loop_callback():  There were two definitions of this function.
Deleted one, changed the other to use enumerate().

poll():  This was trying to repair a bug in Python 2.2's asyncore.poll().
2.2 is no longer supported, so removed this poll().

_start_loop(), _stop_loop():  Removed; functionality folded into
reworked loop().

loop():  The signature of asyncore.loop() changed in 2.4, but this
still had "the old" signature.  Instead of sniffing the signature
of the Python in use, this does a saner thing:  the original
asyncore.loop is captured, and called from the body of this loop().
That way, (A) we don't care what asyncore.loop's signature is; and,
(B) we still call the original Python code, so debugging prints and
breakpoints (etc) stuffed into Python's asyncore no longer "vanish
by magic" when ZEO is used.

an OS-indepedent base class.

__repr__:  Use the postive_id function to embed the machine address.
Addresses with the high bit set trigger warnings before Python 2.4,
and come out as negative numbers in 2.4+.

Windows trigger.__init__:  Don't make 50 guesses at a port number
to use, let Windows pick an available port for us.  Also documented
the baffling single-thread socket setup dance, which took an hour to
reverse-engineer (in large part because it used a bare "except" w/ no
clue as to why).

a __del__ method (but probably did at some earlier time).

UndoSearch._readnext():  Add the transaction size to the return value.

sync() call back in to make it clearer.

and increase the sleep time per failing loop iteration.  The hope is
this will allow the test to pass on slower and/or "too busy" machines.
I can't test that, though, since I couldn't get the test to fail at
all on a Windows or a Linux box.

although ZODB doesn't.

the pickle shown is full of unprintable characters.

attribute.

The related txn_mgr spelling of various method arguments is
now deprecated, and a "transaction_manager" spelling is added.