This PR adds a `StatusError` to the `pproto.RowResponse` type, which
allows a stream to pass an error on the stream (encoded into
the `RowResponse.StatusError`). This can be checked downstream
for matching `EOF` or `err != nil` and handled appropriately.
This is helpful mainly with the `RowResponse` reducers which run in
goroutines. Instead of trying to manage a separate channel of errors
from those goroutines, we just follow the grpc model and send the
error with the stream.
In order to standardize results as streams of RowResponse,
this PR introduces two interfaces `StreamClient` and
`StreamServer`) which mirror the grpc stream interfaces.
Upstream users (sqlmapper, vdsm, etc) can implement
instances of these interfaces to ensure that results can
stream through the entire sytem in an expected way.
This PR also fixes a couple of missing data types.
