Memory leak fix on shutdown

[plekakis]

Fix for false-positive memory leak when using leak detection methods (such as CRT dump memory leaks).

Since we cannot rely on static de-initialisation for freeing the registry::s_instance, this is now lazily allocated instead and freed during spdlog::shutdown() function. In my test case, saves ~15 reported memory leaks.

[gabime]

I don’t know. Seems expensive to lock the singleton on each access. Double check locking with barriers could be used, but I never understood it completely. see https://preshing.com/20130930/double-checked-locking-is-fixed-in-cpp11/

[plekakis]

I have updated the function to do double locking. In practice it's simple, you rely on atomics to effectively early out the expensive mutex lock path, if the instance has already been allocated.
The expensive lock path will now only occur once in the lifetime of the logger and not always.

[gabime]

Shouldn’t there be std::atomic_thread_fence(std::memory_order_acquire); and std::atomic_thread_fence(std::memory_order_release); somewhere?

[gabime]

I think this would break the current behavior. Currently, calling shutdown is not mandatory since the registry object would be automatically destroyed. With this pr, the registry will be never destroyed if not calling shutdown.
Please post to the v2.x branch.

[plekakis]

Ah okay, I thought that shutdown was supposed to be called.
Let's close this PR, I will have a look at the v2 branch for now.

[gabime]

@plekakis See #2113 for a discussion and a fix proposal