Fix reading on a closed channel throw UnexpectedEof error

[uuhan]

Stop reading on a closed channel and stop it's keepalive timer.

This may cause an Error after a session closed:

SshError(IO(Custom { kind: UnexpectedEof, error: "early eof" }))

I encountered in my workhorse project, and also find a related issue: #253

After a long trace in the source code, found that this is caused by

1. read data after channel closed:

russh/russh/src/server/mod.rs

Line 903 in 89fed88

let n = cipher::read(&mut stream_read, &mut buffer, &mut *cipher).await?;

russh/russh/src/cipher/mod.rs

Line 254 in 89fed88

stream.read_exact(&mut len).await?;

(insufficient data to be read)
this caused the "early eof" issue.

2. still doing keepalive after channel closed:

russh/russh/src/server/session.rs

Line 525 in 89fed88

self.keepalive_request()?;

this caused the "broken pipe" issue.