ZOOKEEPER-2907: Logged request buffer isn't useful

As per [ZOOKEEPER-2907](https://issues.apache.org/jira/browse/ZOOKEEPER-2907), we have no padding while converting byte to hex and the request type is not part of the output. So I made the following changes:
- Used String formatter to convert byte to hex with padding.
- Added request type as part of the log message.

Please let me know if additional changes are required.

Author: Nishanth Entoor <entoor.nishanth@gmail.com>

Reviewers: HorizonNet <jan.hentschel@ultratendency.com>, maoling <maoling@apache.org

Closes #1221 from nishanth-entoor/ZOOKEEPER-2907

zookeeper-server/src/main/java/org/apache/zookeeper/server/FinalRequestProcessor.java

@@ -595,9 +595,9 @@ public class FinalRequestProcessor implements RequestProcessor {
             ByteBuffer bb = request.request;
             bb.rewind();
             while (bb.hasRemaining()) {
-                sb.append(Integer.toHexString(bb.get() & 0xff));
+                sb.append(String.format("%02x", (0xff & bb.get())));
             }
-            LOG.error("Dumping request buffer: 0x{}", sb.toString());
+            LOG.error("Dumping request buffer for request type {}: 0x{}", Request.op2String(request.type), sb);
             err = Code.MARSHALLINGERROR;
         }
 

zookeeper-server/src/main/java/org/apache/zookeeper/server/PrepRequestProcessor.java

@@ -955,13 +955,13 @@ public class PrepRequestProcessor extends ZooKeeperCriticalThread implements Req
             if (bb != null) {
                 bb.rewind();
                 while (bb.hasRemaining()) {
-                    sb.append(Integer.toHexString(bb.get() & 0xff));
+                    sb.append(String.format("%02x", (0xff & bb.get())));
                 }
             } else {
                 sb.append("request buffer is null");
             }
 
-            LOG.error("Dumping request buffer: 0x{}", sb.toString());
+            LOG.error("Dumping request buffer for request type {}: 0x{}", Request.op2String(request.type), sb);
             if (request.getHdr() != null) {
                 request.getHdr().setType(OpCode.error);
                 request.setTxn(new ErrorTxn(Code.MARSHALLINGERROR.intValue()));