Port FSTLevelDBQueryCache to C++ (#2202)

* Port FSTLevelDBQueryCache to C++

Author: Greg Soltis

Firestore/Source/Local/FSTLevelDB.mm

@@ -345,6 +345,7 @@ - (instancetype)initWithLevelDB:(std::unique_ptr<leveldb::DB>)db
         [[FSTLevelDBLRUDelegate alloc] initWithPersistence:self lruParams:lruParams];
     _transactionRunner.SetBackingPersistence(self);
     _users = std::move(users);
+    // TODO(gsoltis): set up a leveldb transaction for these operations.
     [_queryCache start];
     [_referenceDelegate start];
   }

Firestore/Source/Local/FSTLevelDBQueryCache.h

@@ -38,12 +38,6 @@ NS_ASSUME_NONNULL_BEGIN
  */
 + (nullable FSTPBTargetGlobal *)readTargetMetadataFromDB:(leveldb::DB *)db;
 
-/**
- * Retrieves the global singleton metadata row using the given transaction, if it exists.
- */
-+ (nullable FSTPBTargetGlobal *)readTargetMetadataWithTransaction:
-    (firebase::firestore::local::LevelDbTransaction *)transaction;
-
 - (instancetype)init NS_UNAVAILABLE;
 
 /**