[JNI] Each set function of Options / WriteOptions now returns its option instance.

Summary:
Make each set function of Options / WriteOptions return its option instance.
Java developers can now easier specify each option like the following:

    options.setCreateIfMissing(true)
        .setWriteBufferSize(8 * 1024)
        .setMaxWriteBufferNumber(3)
        .setDisableSeekCompaction(true)
        .setBlockSize(64 * 1024)
        .setMaxBackgroundCompactions(10);

Test Plan:
make rocksdbjava
make jtest

Reviewers: haobo, ankgup87, sdong, dhruba

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17661

java/org/rocksdb/WriteOptions.java

@@ -40,9 +40,14 @@ public class WriteOptions {
    * system call followed by "fdatasync()".
    *
    * Default: false
+   *
+   * @param flag a boolean flag to indicate whether a write
+   *     should be synchronized.
+   * @return the instance of the current WriteOptions.
    */
-  public void setSync(boolean flag) {
+  public WriteOptions setSync(boolean flag) {
     setSync(nativeHandle_, flag);
+    return this;
   }
 
   /**
@@ -68,9 +73,14 @@ public class WriteOptions {
   /**
    * If true, writes will not first go to the write ahead log,
    * and the write may got lost after a crash.
+   *
+   * @param flag a boolean flag to specify whether to disable
+   *     write-ahead-log on writes.
+   * @return the instance of the current WriteOptions.
    */
-  public void setDisableWAL(boolean flag) {
+  public WriteOptions setDisableWAL(boolean flag) {
     setDisableWAL(nativeHandle_, flag);
+    return this;
   }
 
   /**
@@ -92,5 +102,5 @@ public class WriteOptions {
   private native boolean disableWAL(long handle);
   private native void dispose0(long handle);
 
-  protected  long nativeHandle_;
+  protected long nativeHandle_;
 }