1. Fixed a synchronization issue with database update processing and expiry tracking.
2. Fixed a synchronization issue with MapDBLog appendEntries method.
3. DatabaseClient now uses ProtocolClient to interact with Raft cluster.
4. Misc javdoc and logging improvements

Change-Id: I147eb5bf859cf9827df452d62ab415d643a00aa4
diff --git a/core/api/src/main/java/org/onlab/onos/store/service/DatabaseException.java b/core/api/src/main/java/org/onlab/onos/store/service/DatabaseException.java
index bbc2daf..1468d1b 100644
--- a/core/api/src/main/java/org/onlab/onos/store/service/DatabaseException.java
+++ b/core/api/src/main/java/org/onlab/onos/store/service/DatabaseException.java
@@ -19,4 +19,18 @@
 
     public DatabaseException() {
     };
+
+    public static class Timeout extends DatabaseException {
+        public Timeout(String message, Throwable t) {
+            super(message, t);
+        }
+
+        public Timeout(String message) {
+            super(message);
+        }
+
+        public Timeout(Throwable t) {
+            super(t);
+        }
+    }
 }
\ No newline at end of file