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

CREATE_CHECK_PARENTS controls if the existance of the parent node needs
to be checked during the creation of a child node.

Example: The check prevents the creation of file `/parent/child`
if `/parent` doesn't exists or if it is not a directory

By default CREATE_CHECK_PARENTS is enable and the check is enforced.
It can be disable to speedup creation operations by passing
`-DCREATE_CHECK_PARENTS:BOOL=OFF` as cmake parameter.

lseek was triggering a number overflow by converting long to integer on
its return value

Added specific lseek test

CHECK_ACCESS didn't make any sense since internally we don't support uid/gid

DO_LOOKUP were not used at all

The access RPC have been removed. The POSIX access functionality now is implemented through the stat RPC. This makes code easier to maintain and modify.

There would be a little performance degradation on the POSIX access
function because we are now retrieving also the metadata
associated to the file.

This change makes a lot easier to add symlink support

 - improved statfs system call
 - intercepted statvfs library call

Both of the above function now report the aggregated real size
of all the data-nodes.

You can test this with `df "/tmp/gekkofs_mountpoint"` command

 - Added adafs_pwrite and adafs_pread functions:
        they accept an OpenFile pointer instead of a fd so that can be
        called by function that already accessed the OpenFileMap.

 - Moved the writev function into internal adafs functions.

 - Added internal adafs_write function:
        instead messing up with the file position at the interception
        layer, we can use this internal function that hides the update of the
        file position two.

Can be used to retrieve the Metadata object for a named path

In several places the adafs_stat function have been replaced with this
new adafs_metadata. This allowed to avoid the construnction of the
additional `struct stat`.