Make store/cache thread safe

All the existing stores/cache need to be thread safe in order to be used by multithreaded tasks. The following changes are made to ensure the thread safety of the stores:

• For CachedStore, use sychronized lock for each public function;
• For current InMemoryKeyValueStore, use ConcurrentSkipListMap as underlying map for thread safety.
• For store Iterator, do not support remove functionality (throw UnsupportedOperationException like RocksDb does today).