Multithreading support

[Spxg]

Now if you use sqlite in a child thread, it will panic because extern_ref is not shared in the thread. See rustwasm/wasm-bindgen#955.

The current solution is to use fragile to determine whether it is a child thread. If it is a child thread, the data will be sended to the main thread for execution (via channel maybe).

More design is needed at present.