This change adds a new method, setLocalTransaction(), to the
Connection class.  ZODB applications can call this method to bind
transactions to connections rather than threads.  This is especially
useful for GUI applications, which often have only one thread but
multiple independent activities within that thread (generally one per
window).  Thanks to Christian Reis for championing this feature.

Applications that take advantage of this feature should not use the
get_transaction() function.  Until now, ZODB itself sometimes assumed
get_transaction() was the only way to get the transaction.  Minor
corrections have been added.  The ZODB test suite, on the other hand,
can continue to use get_transaction(), since it is free to assume that
transactions are bound to threads.