loading index data.  This leverages the new fsBucket toString and
fromString methods and provides much faster FileStorage index saving and loading
and smaller index files.  On my machine, saves are 5 times faster and
loads are 20 times faster (after a save, when data are in disk
cache).  Indexes are roughly 30% smaller.

The index format has changed.  Old indexes can be read just fine, but
new indexes won't be readable by older versions of ZODB.