Fix 30s websocket timeout for shiny gxits

A fix for a problem found with @LainPavot-Inrae: shiny gxits (e.g. xseeker) freeze after 30s of inactivity. It turns out it's nginx that kills the websocket when no data is transferred in 30 seconds by default. Changing the timeout to 1 day, which may seem too much but 1/ shiny doc recommends 20 days 2/ the connexion will most probably never reach this1 day limit (supposes browser remains open 1d with no interruption at all) 3/ it's only for gxit traffic which is quite small

Also setting proxy_buffering to off as recommended by shiny doc