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_builder.cc

@@ -77,6 +77,14 @@ void LogPropertiesCollectionError(
 
 }  // anonymous namespace
 
+// kBlockedBasedTableMagicNumber was picked by running
+//    echo http://code.google.com/p/leveldb/ | sha1sum
+// and taking the leading 64 bits.
+// Please note that kBlockedBasedTableMagicNumber may also be accessed by
+// other .cc files so it have to be explicitly declared with "extern".
+extern const uint64_t kBlockedBasedTableMagicNumber
+    = 0xdb4775248b80fb57ull;
+
 struct BlockBasedTableBuilder::Rep {
   Options options;
   WritableFile* file;
@@ -503,7 +511,7 @@ Status BlockBasedTableBuilder::Finish() {
 
   // Write footer
   if (ok()) {
-    Footer footer;
+    Footer footer(kBlockedBasedTableMagicNumber);
     footer.set_metaindex_handle(metaindex_block_handle);
     footer.set_index_handle(index_block_handle);
     std::string footer_encoding;