rgw/rest: add 'zero' rest api

rgw/rest: add 'zero' rest api

when 'zero' is included in rgw_enable_apis, a /zero endpoint is added.
all paths under /zero refer to a single resource that supports the
following operations:

PUT requests discard the request body and update the content length
GET requests return a response body full of zeroes
HEAD requests return the current content length
DELETE requests reset the content length to 0

none of these operations require any authentication or i/o, so can be
used to benchmark the http frontend in isolation

Signed-off-by: Casey Bodley <cbodley@redhat.com>