Change X-Parsoid-Performance header value indicating cached response

The header should comprise 'key=value; ' pairs rather than 'key: value', and
the key name 'cached-response' is less tied to the particulars of the current
Parsoid storage setup (which is being revised). Follows I8085fdba3.

Change-Id: I8bcf947763512ba700793974ab0c48d65c171bbb