Add ownership checkings for FlagLock under debug (#432)

* add ownership checkings for FlagLock under debug

Signed-off-by: SchrodingerZhu <i@zhuyi.fan>

* fix owner reset ordering

Signed-off-by: SchrodingerZhu <i@zhuyi.fan>

* addresss CR

Signed-off-by: SchrodingerZhu <i@zhuyi.fan>

* fix include and constexpr problem

Signed-off-by: SchrodingerZhu <i@zhuyi.fan>

* use thread_local variable as thread identity

Signed-off-by: SchrodingerZhu <i@zhuyi.fan>

* change default form for initialization

Signed-off-by: SchrodingerZhu <i@zhuyi.fan>

* address CR

Signed-off-by: SchrodingerZhu <i@zhuyi.fan>

* fix typo and format

Signed-off-by: SchrodingerZhu <i@zhuyi.fan>

* add more assertions

Signed-off-by: SchrodingerZhu <i@zhuyi.fan>

* adjust flag lock and comments

Signed-off-by: SchrodingerZhu <i@zhuyi.fan>

* address CR

Signed-off-by: SchrodingerZhu <i@zhuyi.fan>

src/mem/pool.h

@@ -29,7 +29,7 @@ namespace snmalloc
 
   private:
     MPMCStack<T, PreZeroed> stack;
-    std::atomic_flag lock = ATOMIC_FLAG_INIT;
+    FlagWord lock{};
     T* list{nullptr};
 
   public: