avoid deadlock: cancel hydration requests coming from itself

avoid deadlock: cancel hydration requests coming from itself

our current infrastructure will cause a deadlock for any hydration
requests coming from desktop files client itself

the main thread is responsible for executing the hydration request but
if the same thread is already blocked waiting for the hydration to
happen, both (open system call and hydration request handling) process
will never be completed and wait for each over in a stuck cycle

Signed-off-by: Matthieu Gallien <matthieu.gallien@nextcloud.com>