GetUpdatesSince API to enable replication.

Summary:
How it works:
* GetUpdatesSince takes a SequenceNumber.
* A LogFile with the first SequenceNumber nearest and lesser than the requested Sequence Number is found.
* Seek in the logFile till the requested SeqNumber is found.
* Return an iterator which contains logic to return record's one by one.

Test Plan:
* Test case included to check the good code path.
* Will update with more test-cases.
* Feedback required on test-cases.

Reviewers: dhruba, emayanke

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D7119

db/transaction_log_iterator_impl.h

@@ -0,0 +1,66 @@
+// Copyright 2008-present Facebook. All Rights Reserved.
+#ifndef STORAGE_LEVELDB_INCLUDE_WRITES_ITERATOR_IMPL_H_
+#define STORAGE_LEVELDB_INCLUDE_WRITES_ITERATOR_IMPL_H_
+
+#include <vector>
+
+#include "leveldb/env.h"
+#include "leveldb/options.h"
+#include "leveldb/types.h"
+#include "leveldb/transaction_log_iterator.h"
+#include "db/log_file.h"
+#include "db/log_reader.h"
+
+namespace leveldb {
+
+struct LogReporter : public log::Reader::Reporter {
+  Env* env;
+  Logger* info_log;
+  uint64_t log_number;
+  virtual void Corruption(size_t bytes, const Status& s) {
+    Log(info_log, "%ld: dropping %d bytes; %s",
+        log_number, static_cast<int>(bytes), s.ToString().c_str());
+  }
+};
+
+class TransactionLogIteratorImpl : public TransactionLogIterator {
+ public:
+  TransactionLogIteratorImpl(const std::string& dbname,
+                     const Options* options,
+                     SequenceNumber& seqNum,
+                     std::vector<LogFile>* files);
+  virtual ~TransactionLogIteratorImpl() {
+    //  TODO move to cc file.
+    if (currentLogReader_ != NULL) {
+      delete currentLogReader_;
+    }
+    delete files_;
+  }
+
+  virtual bool Valid();
+
+  virtual void Next();
+
+  virtual Status status();
+
+  virtual void GetBatch(WriteBatch* batch);
+
+ private:
+  const std::string& dbname_;
+  const Options* options_;
+  const uint64_t sequenceNumber_;
+  const std::vector<LogFile>* files_;
+  bool started_;
+  bool isValid_;  // not valid when it starts of.
+  Status currentStatus_;
+  size_t currentFileIndex_;
+  Slice currentRecord_;
+  log::Reader* currentLogReader_;
+  Status OpenLogFile(const LogFile& logFile, SequentialFile** file);
+  LogReporter NewLogReporter(uint64_t logNumber);
+};
+
+
+
+}  //  namespace leveldb
+#endif  //  STORAGE_LEVELDB_INCLUDE_WRITES_ITERATOR_IMPL_H_