[FIX] web: AbstractController: handle concurrent reloads

Let's assume the following scenario in a Kanban view with a default
filter. The user removes the filter, and quickly adds a new one.
On a slow network, it may happen that the second reload request
returns before the first one. On odoo.com, this is easy to
reproduce on the tasks Kanban view.

Before this rev., when this occured, the result of the first
request was displayed, whereas it should have been ignored as
another request was done later on.