mon/MonClient: check stopping for auth request handling

mon/MonClient: check stopping for auth request handling

When the MonClient is shutting down, it is no longer safe to
access MonClient::auth and other members. The AuthClient
methods should be checking the stopping flag in this case.

The key bit from the segfault backtrace (thanks Brad Hubbard!) is here:

	#13 0x00007f921ee23c40 in ProtocolV2::handle_auth_done (this=0x7f91cc0945f0, payload=...) at /usr/include/c++/12/bits/shared_ptr_base.h:1665
	#14 0x00007f921ee16a29 in ProtocolV2::run_continuation (this=0x7f91cc0945f0, continuation=...) at msg/./src/msg/async/ProtocolV2.cc:54
	#15 0x00007f921edee56e in std::function<void (char*, long)>::operator()(char*, long) const (__args#1=0, __args#0=<optimized out>, this=0x7f91cc0744d8) at /usr/include/c++/12/bits/std_function.h:591
	#16 AsyncConnection::process (this=0x7f91cc074140) at msg/./src/msg/async/AsyncConnection.cc:485
	#17 0x00007f921ee3300c in EventCenter::process_events (this=0x55efc9d0a058, timeout_microseconds=<optimized out>, working_dur=0x7f921a891d88) at msg/./src/msg/async/Event.cc:465
	#18 0x00007f921ee38bf9 in operator() (__closure=<optimized out>) at msg/./src/msg/async/Stack.cc:50
	#19 std::__invoke_impl<void, NetworkStack::add_thread(Worker*)::<lambda()>&> (__f=...) at /usr/include/c++/12/bits/invoke.h:61
	#20 std::__invoke_r<void, NetworkStack::add_thread(Worker*)::<lambda()>&> (__fn=...) at /usr/include/c++/12/bits/invoke.h:111
	#21 std::_Function_handler<void(), NetworkStack::add_thread(Worker*)::<lambda()> >::_M_invoke(const std::_Any_data &) (__functor=...) at /usr/include/c++/12/bits/std_function.h:290
	#22 0x00007f921e81f253 in std::execute_native_thread_routine (__p=0x55efc9e9c5f0) at ../../../../../src/libstdc++-v3/src/c++11/thread.cc:82
	#23 0x00007f921f5e8ac3 in start_thread (arg=<optimized out>) at ./nptl/pthread_create.c:442
	#24 0x00007f921f67a8d0 in clone3 () at ../sysdeps/unix/sysv/linux/x86_64/clone3.S:81

I originally thought this may be the issue causing [1] however that
turned out to be an issue caused by OpenSSL's use of atexit handlers.

I still think there is a bug here so I am continuing with this change.

[1] https://tracker.ceph.com/issues/59335

Fixes: https://tracker.ceph.com/issues/76017
Signed-off-by: Patrick Donnelly <pdonnell@ibm.com>