Legacy certificate signer shims

These request shims adapt legacy style certificate signer requests to work with the new API.

1. First request: POST upload a certificate signing request
    1. The client uploads the CSR.
    2. The server creates the certificate and caches it (with 1 minute TTL).
2. Second request: GET the certificate
    1. The client retrieves the certificate.
    2. The certificate is deleted from the server.