It was impossible to commit a transaction that involved a blob that hadn't
had its 'open' method called, because the blob filename is computed during open.

 - added small non-yet-working work-around

the ZODB_BLOB_TEMPDIR environment variable.  We don't use a value from ZConfig
because ZConfig use isn't mandated for standalone ZODB usage.

 - made ClientStorage and BlobStorage create the blob root directories automatically

 - replaced "DIRTY" with "LOADING" for ClientStorage

 - fixed problem handling data manager registrations for non-default 
   transaction managers

Formatting changes.

- Made the ClientStorage cache directory the same structure as BlobStorage

up on the system. Also add some tests for the configuration of BlobStorage.