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

db/write_batch_test.cc

@@ -22,7 +22,7 @@ namespace rocksdb {
 static std::string PrintContents(WriteBatch* b) {
   InternalKeyComparator cmp(BytewiseComparator());
   auto factory = std::make_shared<SkipListFactory>();
-  MemTable* mem = new MemTable(cmp, factory);
+  MemTable* mem = new MemTable(cmp, factory.get());
   mem->Ref();
   std::string state;
   Options options;