Have API pass through 'X-Parsoid-Performance' header from Parsoid

Change I1d7fd94f2 to Parsoid adds a custom header on Parsoid HTTP responses,
'X-Parsoid-Performance', which contains data about how long it took Parsoid to
handle the request. This patch makes the API pass through the header from
Parsoid to the client, where it will be logged as an event (along with
additional datapoints that are only available on the client). When the request
is fulfilled entirely by Varnish, the XPP value is replaced with
'backend-varnish-hit: true'.

Change-Id: I8085fdba35c860eaaa4df020d06d1d45d4ea5813
Merge "Have API pass through 'X-Parsoid-Performance' header from Parsoid"