Fix a bug in table builder

Summary:
In talbe.cc, when reading the metablock, it uses BytewiseComparator();
However in table_builder.cc, we use r->options.comparator. After tracing
the creation of r->options.comparator, I found this comparator is an
InternalKeyComparator, which wraps the user defined comparator(details
can be found in DBImpl::SanitizeOptions().

I encountered this problem when adding metadata about "bloom filter"
before. With different comparator, we may fail to do the binary sort.

Current code works well since there is only one entry in meta block.

Test Plan:
make all check

I've also tested this change in https://reviews.facebook.net/D8283 before.

Reviewers: dhruba, haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D13335

table/table_builder.cc

@@ -269,7 +269,11 @@ Status TableBuilder::Finish() {
 
   // Write metaindex block
   if (ok()) {
-    BlockBuilder meta_index_block(&r->options);
+    // We use `BytewiseComparator` as the comparator for meta block.
+    BlockBuilder meta_index_block(
+        r->options.block_restart_interval,
+        BytewiseComparator()
+    );
     if (r->filter_block != nullptr) {
       // Add mapping from "filter.Name" to location of filter data
       std::string key = "filter.";