[Performance Branch] PlainTable to encode rows with seqID 0, value type using 1 internal byte.

Summary: In PlainTable, use one single byte to represent 8 bytes of internal bytes, if seqID = 0 and it is value type (which should be common for bottom most files). It is to save 7 bytes for uncompressed cases.

Test Plan: make all check

Reviewers: haobo, dhruba, kailiu

Reviewed By: haobo

CC: igor, leveldb

Differential Revision: https://reviews.facebook.net/D15489

util/testutil.h

@@ -8,6 +8,8 @@
 // found in the LICENSE file. See the AUTHORS file for names of contributors.
 
 #pragma once
+#include <string>
+#include "db/dbformat.h"
 #include "rocksdb/env.h"
 #include "rocksdb/slice.h"
 #include "util/random.h"
@@ -51,5 +53,28 @@ class ErrorEnv : public EnvWrapper {
   }
 };
 
+// An internal comparator that just forward comparing results from the
+// user comparator in it. Can be used to test entities that have no dependency
+// on internal key structure but consumes InternalKeyComparator, like
+// BlockBasedTable.
+class PlainInternalKeyComparator : public InternalKeyComparator {
+ public:
+  explicit PlainInternalKeyComparator(const Comparator* c)
+      : InternalKeyComparator(c) {}
+
+  virtual ~PlainInternalKeyComparator() {}
+
+  virtual int Compare(const Slice& a, const Slice& b) const override {
+    return user_comparator()->Compare(a, b);
+  }
+  virtual void FindShortestSeparator(std::string* start,
+                                     const Slice& limit) const override {
+    user_comparator()->FindShortestSeparator(start, limit);
+  }
+  virtual void FindShortSuccessor(std::string* key) const override {
+    user_comparator()->FindShortSuccessor(key);
+  }
+};
+
 }  // namespace test
 }  // namespace rocksdb