Refine lock/unlock shared memory (#2682)

Split memory instance's field `uint32 ref_count` into `bool is_shared_memory`
and `uint16 ref_count`, and lock the memory only when `is_shared_memory`
flag is true, no need to acquire a lock for non-shared memory when shared
memory feature is enabled.

core/iwasm/interpreter/wasm_runtime.c

@@ -374,6 +374,7 @@ memory_instantiate(WASMModuleInstance *module_inst, WASMModuleInstance *parent,
 
 #if WASM_ENABLE_SHARED_MEMORY != 0
     if (is_shared_memory) {
+        memory->is_shared_memory = true;
         memory->ref_count = 1;
     }
 #endif