Change header_features constant to a function in concurrent_hash_map.

header_features was only declared (not defined) in concurrent_hash_map header.
This means that (until C++17) it can be used only in places where address of
this constant is not needed. In operator != we access one member of header_fatures
which requrires it to be declared. This only fails for some compilers like clang-9
because rest of the evaluate this expression at compile time (this is a compiler
decision whether to do this at compile time or runtime).

Fix this issue by creating function header_features().