TransactionLogIter should stall at the last record. Currently it errors out

Summary:
* Add a method to check if the log reader is at EOF.
* If we know a record has been flushed force the log_reader to believe it is not at EOF, using a new method UnMarkEof().

This does not work with MMpaed files.

Test Plan: added a unit test.

Reviewers: dhruba, heyongqiang

Reviewed By: heyongqiang

CC: leveldb

Differential Revision: https://reviews.facebook.net/D9567

db/log_reader.h

@@ -59,6 +59,17 @@ class Reader {
   // Undefined before the first call to ReadRecord.
   uint64_t LastRecordOffset();
 
+  // returns true if the reader has encountered an eof condition.
+  bool IsEOF() {
+    return eof_;
+  }
+
+  // when we know more data has been written to the file. we can use this
+  // function to force the reader to look again in the file.
+  void UnmarkEOF() {
+    eof_ = false;
+  }
+
   SequentialFile* file() { return file_.get(); }
 
  private: