Add Memtable Read Tier to RocksJava

Summary:
This options was introduced in the C++ API in #1953 .
Closes https://github.com/facebook/rocksdb/pull/3064

Differential Revision: D6139010

Pulled By: sagar0

fbshipit-source-id: 164de11d539d174cf3afe7cd40e667049f44b0bc

java/src/main/java/org/rocksdb/ReadTier.java

@@ -11,7 +11,8 @@ package org.rocksdb;
 public enum ReadTier {
   READ_ALL_TIER((byte)0),
   BLOCK_CACHE_TIER((byte)1),
-  PERSISTED_TIER((byte)2);
+  PERSISTED_TIER((byte)2),
+  MEMTABLE_TIER((byte)3);
 
   private final byte value;