[ticket/16891] Do not change an extension status in the middle of a request
When enabling an extension, it should be considered as not being enabled for
the entire request as if the "enabled" flag is switch during the request, the
extension will stay not loaded (same when disabling an extension).
Example when it can be an issue today : if the router is called for the first
time after enabling the extension and if the extension uses a custom route
loader (like phpbb/pages) then the router will fail because the custom route
will be found but the custom loader will not be loaded and available.
