SshDaemon: Install unknown channel reference handler
SshDaemon: Install unknown channel reference handler

Since SSHD 1.7.0 unknown channel reference should be provided, otherwise
session can crash, see this SSHD issue for more details: [1].

[1] https://issues.apache.org/jira/browse/SSHD-942

Bug: Issue 11491
Change-Id: Ic3d5f86ead0cf9669a3d73d6822ccd8b86114607

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