Add 'sessionId' into privet/info

Current implementation is a Unix timestamp plus counter.

BUG=26140026

Change-Id: Idfe0aa81c49e6dab5d638cbedfbeb460b70b5864
Reviewed-on: https://weave-review.googlesource.com/2010
Reviewed-by: Alex Vakulenko <avakulenko@google.com>
8 files changed