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.h

@@ -79,8 +79,11 @@ typedef union {
 struct WASMMemoryInstance {
     /* Module type */
     uint32 module_type;
+
+    bool is_shared_memory;
+
     /* Shared memory flag */
-    bh_atomic_32_t ref_count; /* 0: non-shared, > 0: reference count */
+    bh_atomic_16_t ref_count; /* 0: non-shared, > 0: reference count */
 
     /* Number bytes per page */
     uint32 num_bytes_per_page;