Implement feature flags in the concurrent skip list

[vinser52]

It would be good to reserve some space + add feature flags similarly as we have in concurrent_hash_map.

The reserved space should be calculated so that hash_map is aligned to 64B (or even, better 256B)

[igchor]

Features flags could be probably implemented in some general way (as a separate type?).

Additionally:

• add layout test
• static assert on std::atomic used in map