TODO

* Add working examples of nginx modules that use protobuf messages.

  - working version of user cookie filter module described in the
    README.md.
  - protobuf RPC module (reads input message from request body of
    a POST, responds with serialized output message).  Can be used
    as an example of what code generation of services would look like.
  - logging module (access logs in protobuf format).  Can write
    compressed logs of {length, message} pairs.  Should include a
    simple utility to parse the gzipped protobuf logs.

* Extend pack/unpack methods to handle incremental pack and unpack.

  - on pack, methods should return NGX_AGAIN if they need more space
    to pack the entire message.
  - on unpack, return NGX_AGAIN if we need more data to finish a field.
    note that it's possible to return NGX_OK even not all of the input
    has been parsed (e.g. if we finish a repeated field exactly at the
    boundary, we will return NGX_OK, but additional input can add more
    fields).
  - state should be saved to / restored from the protobuf context.
    for nested messages, this may include a stack of some kind.
  - caller should check the context to see if the pack or unpack was
    complete when expected to be.

* Add support for default values.

  - when a message is fully unpacked, any missing fields with default
    values will be initialized to those values.

* Consider how / whether to handle services in the .proto files.  from
  the protobuf documentation:

  service SearchService {
    rpc Search (SearchRequest) returns (SearchResponse);
  }

  this is supposed to define an RPC interface that takes a SearchRequest
  and responds with a SearchResponse.  in nginx, we could generate a
  SearchService (ngx_search_service) module that has a handler in it
  that expects a POST of a serialized ngx_search_request_t object, and
  responds with a serialized ngx_search_response_t object.  the module
  can take care of everything except filling in the actual body of this
  function:

  ngx_search_response_t *
  ngx_search_service_handler(ngx_search_request_t *req);

  in the nginx config, the handler would be invoked as:

  location = /searchService {
    ngx_search_service;
  }

  or something along those lines.