pirserver/protocol

The tor process will launch the pirserver process, and hold pipes to the
pirserver's stdin, stdout, and stderr.  All communication between the
processes is over those pipes.  Anything the pirserver outputs over the
stderr pipe should be logged by the tor process.  If the pirserver finds
its stdin has reached EOF, it should terminate.

The general format of request messages (tor -> pirserver) is:

request_id: 8 bytes
request_type: 1 byte
body_len: 4 bytes (big-endian unsigned int)
body: (body_len) bytes

The general format of response messages (pirserver -> tor) is:

request_id: 8 bytes (must match the request_id being responded to)
response_type: 1 byte
body_len: 4 bytes (big-endian unsigned int)
body: (body_len) bytes

There are three request types:

PIRSERVER_REQUEST_PARAMS (0x01)
Request the current pirserver params data
    The body_len must be 0.  The pirserver will respond with
    PIRSERVER_RESPONSE_PARAMS. (TODO: do we want to allow for an error
    response to this request?)

PIRSERVER_REQUEST_STORE (0x02)
Store an object with a given 32-byte key
    The body_len must be at least 32.  The first 32 bytes of the body
    are the key under which to store the object, and the remaining bytes
    are the object.  If body_len is exactly 32, it means to remove the
    object with the specified key.  Storing an object with an
    already-existing key will overwrite the old object with that key.
    This request does not elicit a response from the pirserver.

PIRSERVER_REQUEST_LOOKUP (0x03)
Privately look up an object
    The body_len will depend on the details of the particular private
    lookup scheme (XPIR, ZeroTrace, trivial, etc.).

There are three response types:

PIRSERVER_RESPONSE_PARAMS (0xFF)
Reply with the current pirserver params data
    The body is the params data, which will be sent to the client either
    in an HTTP 200 reply to an explicit params request, or else in a
    special cell piggybacked on the creation of the directory lookup
    circuit.  The body_len will depend on the details on the particular
    private lookup scheme.

PIRSERVER_RESPONSE_LOOKUP_SUCCESS (0xFE)
Response to a successful lookup
    The body is the result of the private lookup requested by the
    corresponding PIRSERVER_REQUEST_LOOKUP.  The body_len will depend on
    the details on the particular private lookup scheme.

PIRSERVER_RESPONSE_LOOKUP_FAILURE (0xFD)
Response to a failed lookup
    The body_len must be 0.