Add wasm_module_obtain() to clone wasm_module_t (#1660)

Also add support for wasm_shared_module_t, and add lock for
wasm_module_t related operations.

And add wasm-c-api sample threads, update sample clone.

samples/wasm-c-api/src/threads.wat

@@ -0,0 +1,5 @@
+(module
+  (func $message (import "" "hello") (param i32))
+  (global $id (import "" "id") i32)
+  (func (export "run") (call $message (global.get $id)))
+)