[protocolv2] Handle invalid requests and internal server errors (#203)

## Why

Clients can send invalid requests for many reasons, most commonly due to
backwards incompatible server changes, server should handle those and
send back a stream abort with it.

## What changed

- Introduced `INTERNAL_RIVER_ERROR` code since some of the errors are
invariant violations on the server
- Split out stream request validation from stream handling
- When we see a bad request, we send `INVALID_REQUEST` code with an
abort bit
- Made tracing `createHandlerSpan` accept tracing fields explicitly
instead of a transport message

some more changes that I'll note inline