Disable client idle disconnection

The MediaWiki client cannot tolerate disconnections, due to
https://github.com/nicolasff/phpredis/issues/403

So the normal result of client disconnection is some sort of random
horrible train wreck in the next client call. So it would be best to
minimise the rate of such disconnections.

Bug: 56886
Change-Id: I8c9b13c1da453ba1abe2564268484ef0ae989a41