Parameterize table magic number

Summary:

As we are having different types of tables and they all might share the same structure in block-based table:

[metaindex block]
[index block]
[Footer]

To be able to identify differnt types of tables, we need to parameterize the "magic number" in the `Footer`.

Test Plan:

make check

table/block_based_table_reader.cc

@@ -29,6 +29,8 @@
 
 namespace rocksdb {
 
+extern uint64_t kBlockedBasedTableMagicNumber;
+
 // The longest the prefix of the cache key used to identify blocks can be.
 // We are using the fact that we know for Posix files the unique ID is three
 // varints.
@@ -242,7 +244,7 @@ Status BlockBasedTable::Open(const Options& options,
     return Status::InvalidArgument("file is too short to be an sstable");
   }
 
-  Footer footer;
+  Footer footer(kBlockedBasedTableMagicNumber);
   s = footer.DecodeFrom(&footer_input);
   if (!s.ok()) return s;