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/CMakeLists.txt

@@ -134,6 +134,7 @@ set(EXAMPLES
   memory
   reflect
   table
+  threads
   trap
 )