DP browse cache to have configurable expiration limit

[prioux]

Right now the list of files on browsable DPs are cached for one minute by the controller. Make the timeout configurable, so that when deploying a DP with static read-only data, we can specify something like '6 months' or even 'forever'.

Remove the caching code in the SingualaritySquashDataProvider which was a kludge to cache stuff for six months, for that same reason.