Beta version 1.2.0b was released

Performance improved on 300%
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Implemented feature requests:
- #1929433: Refactore implementation of shared memory allocators.
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Fixed bugz:
- #1983655: allocator class implementation is incorrect
- #1981817: It's impossible to use memory segment as first decorator
- #1981826: Heap and shared segments provides invalid manager_traits
- #1985090: vector_as_allocator works too slow
- #1986144: memory leaks in performance tests
- #1986138: alloc/dealloc tests implemented incorrectly
- #1986130: there is a lot of code duplications in performance tests
- #1985107: allocator does not validate memory manager type