fix: drain the connection socket on a protocol error

[andydunstall]

Fix for #1327.

To avoid the client write failing on a protocol error, meaning they never read the protocol error response, this closes only the servers send side of the connection (to send a FIN to the client but not an RST if the client keeps writing) then drains the client socket. So on the client write will still succeed, then they will receive the error protocol response.

So in #1327, the client now receives panic: Protocol error: invalid multibulk length rather than panic: write tcp 127.0.0.1:42780->127.0.0.1:6379: write: connection reset by peer.

[romange]

@andydunstall please rebase