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)