Provide performance measurements via 'X-Parsoid-Performance' header

Sets a custom header, 'X-Parsoid-Performance', on HTTP responses. It is
composed of '; '-separated key-value pairs representing various internal
performance measurements.

This will be used by Ori's performance logging framework on the client if the
page was a cache miss.

Change-Id: I1d7fd94f236c1759de859922d2a0604631963aa8