Enhancement: locally cache EAD XSLT results for performance

[ctgraham]

Large finding aids can initiate many inline PHP calls and can be a performance hit. An optional local cache of the XSLT results per Drupal site could improve performance.

It is tempting to make an HTML Fedora derivative of the EAD datastream and serve that, but this misses the possibility of serving the same object from multiple Islandora frontends (and thus multiple base urls), and misses the changes which might come from checking at compile time for the existence of related objects in the links.

[ctgraham]

N.b.: large finding aids may naturally exceed the size limit for cached data in default Drupal installs. (1MB for default memcache and mysql packet size configs). Perhaps this should be stored as temporary files instead.