execute_validated_query_async is using std::sync::RwLock

[jxs]

execute_validated_query_async is using std::sync::RwLock which blocks the thread while waiting to read/write, this will probably block the whole runtime if running under tokio's basic_scheduler.

Both tokio and async-std offer async versions of the RwLock, there's also the futures crate Mutex

[tyranron]

@jxs @LegNeato looking at the code in the current master, the described above is totally non-issue. RwLock's locking is used in a single place only in sync function and there is literally no way for the lock guard to live accross any .await point to block the underlying executor. This is totally correct usage of RwLock there (I'd rather prefer even regular Mutex, as there is no read() locking at all), and using any async Mutex/RwLock will just create extra overhead with no benefit at all, as we do not require to block across .await points.