Factor out _dostore() which stores object revisions, since this is a

common operation. ;)  Also, _dostore() automatically pickles the data
argument, since you don't really want anything else.

Lots of new tests:
    checkNonVersionModifiedInVersion()
    checkLoadSerial()
    checkVersionedStoreAndLoad()
    checkVersionedLoadErrors()
    checkConflicts()
    checkVersionLock()
    checkVersionEmpty()
    checkVersions()
    checkAbortVersion()

Skip for the Minimal storage:
    checkVersionedLoadErrors()
    checkVersionLock()
    checkVersionEmpty()
    checkVersions()
    checkAbortVersion()

Override for the Minimal storage (basically check for Unsupported exceptions):
    checkLoadSerial()
    checkVersionedStoreAndLoad()