gkfs::hg_tag -> gkfs::rpc::tag
gkfs::utils -> gkfs::util
gkfs::client -> gkfs::util
gkfs::logging -> gkfs::log

to avoid name clash with rpc types

Reformatted code w.r.t. unifying code styles as of the official
CPPCoreGuidelines by Stroustrup and Sutter

Restructuring code w.r.t. configurations and definitions:
- #defines have been mostly removed from configurations
- a dedicated config file has been added for configurations with constexpr
- past configure file is now only a cmake wrapper
- wrapping global functions into namespaces

Removed all adafs and ifs occurrences. Now called gkfs

Fixes #74.

Closes #73