[C++] Introduce synchronization and container abstractions

[jcking]

In most cases the synchronization and container implementations provided by the C++ standard library are sufficient. However, there are some environments where users may wish to substitute other implementations. One of those is Abseil. By default, the implementations provided by the C++ standard library are used and should be preferred.

[jcking]

@hzeller

[hzeller]

Wasn't there also a std::call_once in ATNDeserializationOptions.cpp and in XPathLexer.cpp ?

[jcking]

Wasn't there also a std::call_once in ATNDeserializationOptions.cpp and in XPathLexer.cpp ?

Whoops, thanks. Forgot about those.

[hzeller]

LGTM
Can you double check and recursive grep for any of the replaced synchronization primitives (std::call_once, std::mutex, shared, unique lock etc) as mixing them might not be a good idea.

Some of the typical singleton initalizations with call_once might be replaced with the following pattern, which is safe since C++11 and arguably easier to read:

Foo* FooSingleton() {
   static Foo* instance = new Foo();
   return instance;
}

[GoToCoding]

Sorry for writing to finished PR.

Are you considering to fully move to abseil types instead of std?
Because this duplicating defines look little strange if you want to use abseil flatmap in more and more places.
[Of course that's just my opinion]