Add the property block for the plain table

Summary:
This is the last diff that adds the property block to plain table.
The format resembles that of the block-based table: https://github.com/facebook/rocksdb/wiki/Rocksdb-table-format

  [data block]
  [meta block 1: stats block]
  [meta block 2: future extended block]
  ...
  [meta block K: future extended block]  (we may add more meta blocks in the future)
  [metaindex block]
  [index block: we only have the placeholder here, we can add persistent index block in the future]
  [Footer: contains magic number, handle to metaindex block and index block]
  <end_of_file>

Test Plan: extended existing property block test.

Reviewers: haobo, sdong, dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14523

table/block_based_table_builder.cc

@@ -387,7 +387,7 @@ Status BlockBasedTableBuilder::Finish() {
           &properties_block_handle
       );
 
-      meta_index_builer.Add(BlockBasedTable::kPropertiesBlock,
+      meta_index_builer.Add(kPropertiesBlock,
                             properties_block_handle);
     }  // end of properties block writing
 
@@ -459,7 +459,5 @@ uint64_t BlockBasedTableBuilder::FileSize() const {
 
 const std::string BlockBasedTable::kFilterBlockPrefix =
     "filter.";
-const std::string BlockBasedTable::kPropertiesBlock =
-    "rocksdb.properties";
 
 }  // namespace rocksdb