RATIS-2232. Improve log for debugging on RaftLog / TransactionManager (#1203)

Author: SzyWilliam

ratis-server-api/src/main/java/org/apache/ratis/server/raftlog/RaftLogSequentialOps.java

@@ -77,8 +77,10 @@ <OUTPUT, THROWABLE extends Throwable> OUTPUT runSequentially(
         // The current thread is already the runner.
         return operation.get();
       } else {
+        final Throwable cause = new Throwable("The thread already running: " + previous);
+        cause.setStackTrace(previous.getStackTrace());
         throw new IllegalStateException(
-            name + ": Already running a method by " + previous + ", current=" + current);
+            name + ": Already running a method by " + previous + ", current=" + current, cause);
       }
     }
   }

ratis-server/src/main/java/org/apache/ratis/server/impl/TransactionManager.java

@@ -71,12 +71,6 @@ void remove(TermIndex termIndex) {
 
   @Override
   public String toString() {
-    if (contexts.isEmpty()) {
-      return name + " <empty>";
-    }
-
-    final StringBuilder b = new StringBuilder(name);
-    contexts.forEach((k, v) -> b.append("\n  ").append(k).append(": initialized? ").append(v.isInitialized()));
-    return b.toString();
+    return name + ":size=" + contexts.size();
   }
 }