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.