serialization of std::unordered_map does not preserve capacity

[cz4rs]

If the code intentionally enlarges std::unordered_map, serialization code will not preserve its capacity. Note: this affects any UnorderedAssociativeContainer.

• first reported and fixed with Serialization of vector and unordered_map does not preserve capacity #162
• this in turn has introduced a new bug, memory explosion (reported and partially fixed with std::unordered_map serializer causing huge memory explosion with empty container #180)
• the code remained problematic in terms of memory (ElementStats unordered_map capacity keeps growing vt#1304) and has caused Intel compilers to crash (intel builds fail with internal compiler errors vt#1302), resulting in total revert (std::unordered_map serializer increasing memory consumption and causing Intel ICEs #182 )

This can be re-implemented after DARMA-tasking/vt#1302 is fixed. See 9148400 for previous solution.