Cache content is required to be serializable

The values of the document and doctype cache are not serializable, but this is a requirement for clustering.

See https://developer.atlassian.com/display/CONFDEV/Atlassian+Cache+2+Overview:

"Cluster cache keys and values are stored by value, which means they are serialized on insertion to the cache, and deserialized on extraction.

Therefore, to ensure that the data can be shared, both product and plugin developers must ensure that the objects placed into a cluster cache implement serialization with support for versioning."