To remove the code duplication between the host backends.

[MikeDvorskiy]

1. class buffer
2. stuff from parallel scan pattern (upsweep, down sweep etc)
3. to try to write abstraction impl layer which contains the all sorting/ merging optimizations.

Originally posted by @rarutyun in https://github.com/oneapi-src/oneDPL/pull/381#discussion_r734034435