message: Make getter/setter of thread-local variable static
  The function `logger::thread_local_aid` accesses a thread-local
  variable. Hence, dispatching this through to the logger object makes
  little sense.
diff: @@ -220,10 +220,10 @@ public:
  // -- properties -------------------------------------------------------------
  /// Returns the ID of the actor currently associated to the calling thread.
  - actor_id thread_local_aid();
  + static actor_id thread_local_aid();
  /// Associates an actor ID to the calling thread and returns the last value.
  - actor_id thread_local_aid(actor_id aid);
  + static actor_id thread_local_aid(actor_id aid);
  /// Returns whether the logger is configured to accept input for given
  /// component and log level.
  @@ -431,29 +431,20 @@ CAF_CORE_EXPORT bool operator==(const logger::field& x, const logger::field& y);
  if (CAF_UNIFYN(caf_logger) != nullptr \
  && CAF_UNIFYN(caf_logger)->accepts(loglvl, component)) \
  CAF_UNIFYN(caf_logger) \
  - ->log(CAF_LOG_MAKE_EVENT(CAF_UNIFYN(caf_logger)->thread_local_aid(), \
  - component, loglvl, message)); \
  + ->log(CAF_LOG_MAKE_EVENT(caf::logger::thread_local_aid(), component, \
  + loglvl, message)); \
  } while (false)
  #define CAF_PUSH_AID(aarg) \
  - auto CAF_UNIFYN(caf_tmp_ptr) = caf::logger::current_logger(); \
  - caf::actor_id CAF_UNIFYN(caf_aid_tmp) = 0; \
  - if (CAF_UNIFYN(caf_tmp_ptr)) \
  - CAF_UNIFYN(caf_aid_tmp) = CAF_UNIFYN(caf_tmp_ptr)->thread_local_aid(aarg); \
  - auto CAF_UNIFYN(aid_aid_tmp_guard) = caf::detail::make_scope_guard([=] { \
  - auto CAF_UNIFYN(caf_tmp2_ptr) = caf::logger::current_logger(); \
  - if (CAF_UNIFYN(caf_tmp2_ptr)) \
  - CAF_UNIFYN(caf_tmp2_ptr)->thread_local_aid(CAF_UNIFYN(caf_aid_tmp)); \
  - })
  + caf::actor_id CAF_UNIFYN(caf_aid_tmp) = caf::logger::thread_local_aid(aarg); \
  + auto CAF_UNIFYN(caf_aid_tmp_guard) = caf::detail::make_scope_guard( \
  + [=] { caf::logger::thread_local_aid(CAF_UNIFYN(caf_aid_tmp)); })
  #define CAF_PUSH_AID_FROM_PTR(some_ptr) \
  auto CAF_UNIFYN(caf_aid_ptr) = some_ptr; \
  CAF_PUSH_AID(CAF_UNIFYN(caf_aid_ptr) ? CAF_UNIFYN(caf_aid_ptr)->id() : 0)
  -#define CAF_SET_AID(aid_arg) \
  - (caf::logger::current_logger() \
  - ? caf::logger::current_logger()->thread_local_aid(aid_arg) \
  - : 0)
  +#define CAF_SET_AID(aid_arg) caf::logger::thread_local_aid(aid_arg)
  #define CAF_SET_LOGGER_SYS(ptr) caf::logger::set_current_actor_system(ptr)
  