[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

table/filter_block.h

@@ -35,7 +35,8 @@ class FilterPolicy;
 //      (StartBlock AddKey*)* Finish
 class FilterBlockBuilder {
  public:
-  explicit FilterBlockBuilder(const Options& opt);
+  explicit FilterBlockBuilder(const Options& opt,
+                              const Comparator* internal_comparator);
 
   void StartBlock(uint64_t block_offset);
   void AddKey(const Slice& key);