Fix GWT Cache Issue

See: https://app.datadoghq.com/logs?cols=core_host%2Ccore_service&event&from_ts=1571586487983&index=main&live=true&messageDisplay=expanded-md&query=source%3Anuxeo+host%3Anos-prod-%2A+status%3Aerror+IncompatibleRemoteServiceException&saved_view=13914&stream_sort=desc&to_ts=1571672887983

Need to investigate this issue.

First understanding; GWT apps are signed, though when an user reach another version of the app than the one opened on his client side; it might lead to that exception.

Also, we have to keep in mind than any time based cache will lead to this error during a blue/green deploymeny.