Cache the CONTENTdm collection field configuration

[mjordan]

The CdmToMods metadata parser's getCollectionFieldConfig() method makes an HTTP request to $wsUrl . 'dmGetCollectionFieldInfo/' . $alias . '/json' for every MODS record it creates. This could be cached, reducing the number of calls to the CONTENTdm server and also allowing us to create a PHPUnit test for the CdmToMods metadata parser.