Get rid of some shared_ptrs

Summary:
I went through all remaining shared_ptrs and removed the ones that I found not-necessary. Only GenerateCachePrefix() is called fairly often, so don't expect much perf wins.

The ones that are left are accessed infrequently and I think we're fine with keeping them.

Test Plan: make asan_check

Reviewers: dhruba, haobo

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14427

table/block_based_table_builder.cc

@@ -127,9 +127,10 @@ BlockBasedTableBuilder::BlockBasedTableBuilder(
     rep_->filter_block->StartBlock(0);
   }
   if (options.block_cache_compressed.get() != nullptr) {
-    BlockBasedTable::GenerateCachePrefix(options.block_cache_compressed, file,
-                               &rep_->compressed_cache_key_prefix[0],
-                               &rep_->compressed_cache_key_prefix_size);
+    BlockBasedTable::GenerateCachePrefix(
+        options.block_cache_compressed.get(), file,
+        &rep_->compressed_cache_key_prefix[0],
+        &rep_->compressed_cache_key_prefix_size);
   }
 }