Add X-VCAP-Request-ID generation and logging

Proxied requests are assigned a uuid in the 'X-Vcap-Request-Id' that can
be used to help with correlation.

The value of the uuid is captured in the access log.
Update post pull/29 to use the more standard `X-Request-Id`

* also `request_id` instead of `vcap_request` in logs

Signed-off-by: Kai Zhang <kaizhang@cn.ibm.com>