mail-client: Port goa_mail_client_check() to GTask
mail-client: Port goa_mail_client_check() to GTask

This reworks the lifetime of CheckData a little bit, rather than
explicitly freeing it every time the async operation ends with success
or error, CheckData is set a GTask task-data. The GTask is passed around
rather than being contained in CheckData, and is unref'ed every time
CheckData was freed. When the last GTask reference is dropped, the
corresponding CheckData memory will be freed.

https://bugzilla.gnome.org/show_bug.cgi?id=764157

ritzk/gnome-online-accounts,ritzk/gnome-online-accounts,ritzk/gnome-online-accounts