Resolves#1810 by automatically updating the RA ratelimit.RateLimitConfig whenever the backing config file is changed. Much like the Policy Authority uses a reloader instance to support updating the Hostname policy on the fly, this PR changes the Registration Authority to use a reloader for the rate limit policy file.
Access to the ra.rlPolicies member is protected with a RWMutex now that there is a potential for the values to be reloaded while a reader is active.
A test is introduced to ensure that writing a new policy YAML to the policy config file results in new values being set in the RA's rlPolicies instance.
https://github.com/letsencrypt/boulder/pull/1894
* WFE returns headers Boulder-Request-ID and Boulder-Requester
* improve test
* add the requestHeader after calls to verifyPOST
* move call of addRequesterHeader in NewRegistration
* move setting of request header to context, improve test
* remove pointless comment
Ben Irving