Persist error d-bus objects

Use Cereal to implement serialization and de-serialization of
properties of error d-bus objects.

Serialize and persist error d-bus objects as they are put on the bus.
De-serialize and restore them (if persistent ones exist) when
phosphor-log-manager starts up.

Change-Id: I1f5df1abbe74bfdb86e3e82a78ff7115e90e2112
Signed-off-by: Deepak Kodihalli <dkodihal@in.ibm.com>
diff --git a/log_manager.hpp b/log_manager.hpp
index 71b6e71..730f63e 100644
--- a/log_manager.hpp
+++ b/log_manager.hpp
@@ -67,6 +67,11 @@
          */
         void erase(uint32_t entryId);
 
+        /** @brief Construct error d-bus objects from their persisted
+         *         representations.
+         */
+        void restore();
+
     private:
         /** @brief Call metadata handler(s), if any. Handlers may create
          *         associations.