Supvisors

* Fix a random Python crash due to an I/O exception when the Supervisor log file is rolling over.

* Fix a possible race condition in the Supervisor proxies management.

* Completion of fix about process CPU statistics when using SOLARIS mode.

* Add a grid to statistics plots in the **Supvisors** Web UI.

* Fix process CPU statistics when using SOLARIS mode.
Issue introduced in **Supvisors** 0.18 with statistics refactoring.

* Fix `supervisord` uptime in the **Supvisors** Web UI.
Issue introduced in **Supvisors** 0.18 with monotonic time consideration.

* Set a default warning message to state that Supvisors rules don't apply on the Supervisor page
of the **Supvisors** Web UI.

* Emphasize the selected Supervisor instance and Application in the navigation of the **Supvisors** Web UI.

* In discovery mode, clarify the logs in the event where the **Supvisors** nick identifier is not unique.

* Add a section about multicast in documentation troubleshooting.

* Recreate the internal proxy to the local Supervisor if older than 20 minutes, in order to palliate the closure
of all channels inactive during 30 minutes in the Supervisor HTTP server.
This fixes a rare bug that has been introduced in **Supvisors** 0.17, and whose occurrence has greatly
increased after the refactoring of the **Supvisors** 0.18 internal communications.

* Swap Memory and Network statistics cards in the **Supvisors** Web UI.

* Handle the `PermissionError` exception when trying to get disk usage.

* Update CSS for overflow parts in the **Supvisors** Web UI.