Use atomics for pmr get/set default resource

[marksantaniello]

Pmr has the concept of a global "default resource", accessed via get_default_resource/set_default_resource. In the libstdc++ implementation, this is done with std::atomic: https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/include/experimental/memory_resource#L537-L560

In Boost container we have a global lock: https://github.com/boostorg/container/blob/develop/src/global_resource.cpp#L85-L86

The default c'tor for polymorphic_allocator calls get_default_resource, so every default-constructed pmr container will acquire the lock, which is expensive: https://github.com/boostorg/container/blob/b7d48f1e086435cc03edb2a96199c1d226d7221c/include/boost/container/pmr/polymorphic_allocator.hpp#L45-L46

It's extremely common /not/ to pass a fancy allocator to a pmr container at construction time. This is one of the key value propositions of std::pmr -- that it provides "vocabulary types" (such as pmr::vector), which are compile-time compatible and accept fancy allocators on specific instances at runtime.

[igaztanaga]

Many thanks for the report!

[marksantaniello]

Thanks for the fix!