[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

include/rocksdb/options.h

@@ -34,6 +34,7 @@ class TablePropertiesCollector;
 class Slice;
 class SliceTransform;
 class Statistics;
+class InternalKeyComparator;
 
 using std::shared_ptr;
 

include/rocksdb/table.h

@@ -27,8 +27,6 @@
 
 namespace rocksdb {
 
-class TableFactory;
-
 // -- Block-based Table
 class FlushBlockPolicyFactory;