createSessionWithTimeout checked sys.client for nil and then dereferenced
it again inside the goroutine that calls NewSession. update() runs the
SMART fetch in its own goroutine, so closeSSHConnection can clear the
field between those two reads and the goroutine dereferences a nil
client, panicking the whole hub process.
Make client an atomic.Pointer, load it once before starting the
goroutine, and clear it with Swap so a concurrent close cannot be
observed mid-session-creation. NewSession on an already-closed client
returns an error, which the existing retry path already handles.
Closes#2157
The per-system updater reused an SSH client across ticks and ran the data
exchange with no deadline. If a connection went half-open (dead peer that
never sends RST/FIN) or an agent accepted the session but never wrote a
response, the read in fetchDataViaSSH blocked forever. Because
StartUpdater calls update() synchronously on its ticker, a blocked read
froze the whole per-system goroutine: the ticker's subsequent ticks were
dropped, no error was returned so the system stayed "up", and the agent
was never re-dialed until the hub process restarted.
Bound each SSH data exchange with sshOperationTimeout via runWithTimeout:
on timeout the connection is torn down (unwinding the blocked read) and a
retryable error is returned, so the next tick re-dials. Also enable TCP
keep-alive on dialed connections as a backstop for genuine network death.
Fixes#2041
