match libpq treatment of null byte in message header

[rich-murphey]

This is a partial fix for #886. Under a load of 128 concurrent queries, I see 90% fewer failed queries with this change.

This changes the behavior of the driver to match that of libpq. Namely, if a message header starts with a null byte, discard the message and continue without signaling an error.

This is only a partial fix. With this, I still see 1% of queries fail. That's better than 10%, but there's still an issue somewhere.

The issue is not due to dropping PgStream or BufStream. That is not occurring. I instrumented a Drop implementation for them and they are not dropped while the failed queries occur.

[rich-murphey]

I'm going to go ahead and close this. @mehcode explaings the cause of this in #886, suggesting it's due to dropping a future in the middle of recv() of a message.