Allow driver instances to use different threads
Removed unneeded `sqlite3_threadsafe()` check

Since the database is accessed from 1 thread only
thanks to `key` parameter of `driver_async`.
In fact we could previously potentially access the
same DB twice from different emulator threads
precisely when SQLite3 is compiled in single-thread
mode.
Comment fixed
Ignore non-negative driver_async return values
Get rid of a warning on Windows