ssh: return session ID in ConnMeta.SessionID.

SessionID() returned nil previously.

Fixes #9761.

Change-Id: I53d2b347571d21eab2d913c2228e85997a84f757
Reviewed-on: https://go-review.googlesource.com/3872
Reviewed-by: Adam Langley <agl@golang.org>
4 files changed