2nd level cache for CREATERUNTIMEMAP responses

[jng]

For performance, it is worth storing the CREATERUNTIMEMAP response XML content (with templated placeholders for session/mapname/etc) in a 2nd level cache, so that subsequent CREATERUNTIMEMAP requests for the same Map Definition with the same parameters can just read from the 2nd level cache, bypassing the creation of a MgMap and rendering the response XML from it.

Being a representation of a Map Definition, such cached items can be automatically invalidated if the original Map Definition has been updated in the repository.