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>
diff --git a/src/privet/security_delegate.h b/src/privet/security_delegate.h
index 54f957d..d8090bd 100644
--- a/src/privet/security_delegate.h
+++ b/src/privet/security_delegate.h
@@ -60,6 +60,8 @@
 
   virtual bool CancelPairing(const std::string& session_id,
                              ErrorPtr* error) = 0;
+
+  virtual std::string CreateSessionId() = 0;
 };
 
 }  // namespace privet