Retrying HTTP forwarder

This web-service will act as a persistent and retrying queue.
Upon receipt of a POST or PUT-request, the service will asynchronously forward the received HTTP request to a remote host.
When the remote host does not return a success, the request will be retried untill success or untill the retry-scheme is exhausted.
The remote host is indicated by:

• the HTTP query parameeter "HostToForwardTo" or
• the HTTP-request-header "X-HostToForwardTo"

Example request that demonstrates a POST being forwarded to postman-echo.com

   curl -vvv \
        --request POST \
        --data "This is expected to be sent back as part of response body." \
        "https://forwardhttp.appspot.com/post?HostToForwardTo=postman-echo.com&TryFirst=false"