fix: Remove sub changes request when timeout happens

[hiohiohio]

Context:

• putting a request message into c.subChanges blocks process with having c.pendingSubChangeMutex.Lock() when subscribe request is called twice during disconnection
• this leads deadlock to wait c.pendingSubChangeMutex.Lock() in connWriter() when reconnecting

Changes:

• drain c.subChanges channel when timeout happens. expecting this happens only when disconnected as c.subChanges are consumed before the timeout by connWriter() if connected.

[gnvk]

Thank you for fixing this @hiohiohio !