This is required since the EmailThread creation logic strongly rely on the order which Email entities are created

The References header may store a space-separated list of values, which was not correctly handled.

The following three main steps are now implemented in three separate
methods:

- database connection: self.connect(appid)
- importer creation: self.create_importer(cnx)
- mbox file import: self.import_mbox_files(importer, filenames)

The command line helper says that arguments should be
<pyro-appid> <mboxfile1> [<mboxfile2> ...]

Therefore pyro-appid should be the first item of the arguments list,
not the last one.