Use `expires_in` from the oauth2 response
to reset the oauth2_access_token before
it expires
divides the `expires_in` by 2 to ensure
the access_token is cleared before it
expires
ref: https://github.com/OfflineIMAP/offlineimap/issues/536
Signed-off-by: Frode Aannevik <frode.aa@gmail.com>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>