Source code checked in, #40956

Introduce TrySendMessage method on Session which will not let SocketException or SshException exception bubble up, but instead return false in this case. Use TrySendMessage to send SSH_MSG_DISCONNECT, SSH_MSG_CHANNEL_EOF, SSH_MSG_CHANNEL_CLOSE and SSH_MSG_IGNORE messages to avoid throwing exceptions while closing/disposing or sending keep-alive messages. Avoid sending multiple keep-alive messages concurrently for the same client. Fixed regression in multi-factor authentication.