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/auth_manager.h b/src/privet/auth_manager.h
index 44b8bca..a74e43b 100644
--- a/src/privet/auth_manager.h
+++ b/src/privet/auth_manager.h
@@ -55,10 +55,13 @@
   void SetSecret(const std::vector<uint8_t>& secret,
                  RootClientTokenOwner owner);
 
+  std::vector<uint8_t> CreateSessionId();
+
  private:
   Config* config_{nullptr};
   base::DefaultClock default_clock_;
   base::Clock* clock_{&default_clock_};
+  uint32_t session_counter_{0};
 
   std::vector<uint8_t> secret_;
   std::vector<uint8_t> certificate_fingerprint_;