Cache result of ReadFirstRecord()

Summary:
ReadFirstRecord() reads the actual log file from disk on every call. This diff introduces a cache layer on top of ReadFirstRecord(), which should significantly speed up repeated calls to GetUpdatesSince().

I also cleaned up some stuff, but the whole TransactionLogIterator could use some refactoring, especially if we see increased usage.

Test Plan: make check

Reviewers: haobo, sdong, dhruba

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18387

db/db_impl_debug.cc

@@ -117,5 +117,16 @@ Status DBImpl::TEST_WaitForCompact() {
   }
   return bg_error_;
 }
+
+Status DBImpl::TEST_ReadFirstRecord(const WalFileType type,
+                                    const uint64_t number,
+                                    SequenceNumber* sequence) {
+  return ReadFirstRecord(type, number, sequence);
+}
+
+Status DBImpl::TEST_ReadFirstLine(const std::string& fname,
+                                  SequenceNumber* sequence) {
+  return ReadFirstLine(fname, sequence);
+}
 }  // namespace rocksdb
 #endif  // ROCKSDB_LITE