Desallocate the server's thread pool immediately on plugin deactivation

[Pierre-Etienne Poirot]

If the plugin was stopped and restarted multiple times in a row, the thread pool allocated to the server (to serve incoming requests over the Git protocol: git://...) would not be immediately shutdown.

Change this behaviour to shutdown the thread pool immediately instead.

This only affects users that stopped and restarted the plugin multiple times without restarting Stash.