Add support for flagging base map tile cache never to be purged

[zspitzer]

A fully seeded tile cache can take a long tile to generate and can be extremely easily purged via simply updating one of the related layers or feature sources.

There should be an option to set a flag on the map's resource header which prevents any purging of the tile cache by changes to related layers

[tomfukushima]

There should be a single spot where the tile cache is purged by the server. It should be pretty straightforward to add a check to see if the header flag is set. I suggest that if you can find someone to do this (perhaps yourself) that you create an RFC, get it approved, and then submit the patch. I doubt that this will take much effort to code up. Tom.

[jbirch]

Removing version from enhancement request. If this enhancement request has been addressed or is no longer required, please close ticket.