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

include/rocksdb/table_properties.h

@@ -64,6 +64,7 @@ struct TablePropertiesNames {
   static const std::string kFilterPolicy;
 };
 
+extern const std::string kPropertiesBlock;
 
 // `TablePropertiesCollector` provides the mechanism for users to collect
 // their own interested properties. This class is essentially a collection