Global variables in shared libraries are a bad idea. EMailBackend now
owns the MailSession instance, which is actually now EMailSession.
Move the blocking utility functions in mail-tools.c to e-mail-session.c
and add asynchronous variants. Same approach as Camel.
Replace EMailReader.get_shell_backend() with EMailReader.get_backend(),
which returns an EMailBackend. Easier access to the EMailSession.