[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

db/write_batch_test.cc

@@ -57,7 +57,7 @@ static std::string PrintContents(WriteBatch* b) {
         state.append(")");
         count++;
         break;
-      case kTypeLogData:
+      default:
         assert(false);
         break;
     }