GlueSQL Shared Sled Storage

The stock SledStorage does not do concurrency. I added RwLock and transaction state on top of SledStorage to make it as easy to use as the stock SharedMemoryStorage.

• RwLock: maintain each valid transaction, by preventing a new transaction while Sled is locked
• TransactionState: prevent rollback, by avoiding/awaiting new transaction while another transaction that has begin but yet to commit
• Drop: prevent rollback on next instance, by commiting transaction upon drop as graceful termination

Set await_active_transaction as true to await for the active transaction to commit. Set false to just return error when there is active transaction.