In corporate environments, client certificates have short life times
for added security, and they get renewed automatically. This means
that client certificate can expire in the middle of long running
command such as `mount`.
This commit attempts to reload the client certificates 30s before they
expire.
This will be active for all backends which use HTTP.
The purpose of this is to make it easier to maintain and eventually to
allow the rclone backends to be re-used in other projects without
having to use the rclone configuration system.
The new code layout is documented in CONTRIBUTING.
