Merge branch '02_20_code_maintenance' into 'master'

02 20 code maintenance

General code maintenance:
- remove dead code
- remove as many `#define` as possible and use `constexpr` instead
- remove adafs occurrences and replace with gkfs
- reformat code to unify code style as of CPPCoreGuidelines
- rearrange imports, remove unused imports
- separate cmake configurations and FS configurations
- encapsulate non-prefix gkfs functions into namespaces
- change copyright statement to 2020
- cleanup of some irrelevant code and unnecessary struct definitions
- consistent renaming for rpc sender functions and create and remove functions from client to daemon
- encapsulate all code into namespaces even if naming and structuring is tentative

See merge request hpc/gekkofs!31