[Backport-1.6] ER/T half-close logic is incorrect

[plorenz]

Backport #3526 from 2.0 to 1.6

1. ReadPayload should not block in write
2. ReadPayload should only return ErrPeerClosed if the write is closed, not the inverse. The error nil check is switched.
3. ReadPayload should return io.EOF if the write side is still open, not ErrPeerClosed. ErrPeerClosed indicates that both sides of the connection are closed.