Cache zuul status.json for one second.

Serve previously rendered zuul status.json if it is was formatted less
than one second prior. This caches the status.json reducing expensive
calls to format the status.json.

The cache ttl is configurable using the zuul.status_expiry configuration
option in the zuul config.

Change-Id: Ib3fcd75f30bc4d4ef5f09d2adfd566d10c58a991
Merge "Cache zuul status.json for one second."