[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/block_based_table_builder.h

@@ -26,6 +26,7 @@ class BlockBasedTableBuilder : public TableBuilder {
   // building in *file.  Does not close the file.  It is up to the
   // caller to close the file after calling Finish().
   BlockBasedTableBuilder(const Options& options,
+                         const InternalKeyComparator& internal_comparator,
                          WritableFile* file,
                          FlushBlockPolicyFactory* flush_block_policy_factory,
                          CompressionType compression_type);