Locking in encryption threads should be improved

When accessing crypt_data there is an assumption that crypt_data can never change to null when encryption threads are running. That saves lots of locking. However, encryption threads tend to lock fil_system lock a lot, which is quite a performance hit. The locking of encryption threads should be rethought and redesigned - so to at least do not lock fil_system mutex and go with fine-grained shard mutex of space mutex if possible.