use 0L for unknown term

Author: dnhatn

server/src/main/java/org/elasticsearch/index/translog/TranslogHeader.java

@@ -45,7 +45,7 @@ final class TranslogHeader {
     public static final int VERSION_PRIMARY_TERM = 3; // added primary term
     public static final int CURRENT_VERSION = VERSION_PRIMARY_TERM;
 
-    public static final long UNKNOWN_PRIMARY_TERM = -1L;
+    public static final long UNKNOWN_PRIMARY_TERM = 0L;
 
     private final String translogUUID;
     private final long primaryTerm;