Merge branch 'rnou/ExAPI' into 'master'

GekkoFS user library creation.

Build over `replication/gekkofwd` changes. This MR aims to create a standalone library to use inside a client application without `LD_PRELOAD` and `syscall interception`.

The user library needs some changes as some `syscall_no_intercept` calls are scattered through the logging and the general code of the client.
Also we need to do not link `syscall_intercept` and to remove (or avoid) the `gekkofs` client constructor. 

There is an example of `write` - `read` on the example directory.

The user needs to link gkfs_user_lib.so and use user_functions.hpp (installed on include/gkfs directory)

See merge request !171