introduce gitlab CI

See merge request zdvresearch_bsc/adafs!77

margo, argobots and mercury have been updated. The abt-snoozer dep
required by margo it is not needed anymore.

RocksDB has been also updated to the lastest release

Instead of using custom variable to search for installed libraries
we can rely on the standard `CMAKE_PREFIX_PATH` [1]

[1]: https://cmake.org/cmake/help/v3.8/variable/CMAKE_PREFIX_PATH.html#variable:CMAKE_PREFIX_PATH

compile/download scripts need to exit with positive error code on wrong parameters

loggers can be now configured at execution time, by using environment
variables for both server and client.

Server:
 - ADAFS_LOG_LEVEL
 - ADAFS_DAEMON_LOG_PATH

Client:
 - ADAFS_LOG_LEVEL
 - ADAFS_PRELOAD_LOG_PATH

This allows us to quickly see if the file system can provide basic functionality without breaking.

Since we introduced the `initialized` flag in the preload context, it is
not needed anymore to check for ld_is_aux_loaded flag.
In fact ld_is_aux_loaded is true if and only if the `initialized` flag
is true.

The library intercepts IO functions from others library constructors
before our constructor have been executed.

All the interceptions triggered before that our constructor have been
executed are now forwarded to the glibc.

By moving the logger initialization outside of the passthrough
initialization we will be able to implements fopen interceptions.

Avoid the usage of a custom cmake "find-module" for mercury.
Instead use the official exported target

bugfix: wrong termination condition on Metadata::get_dirents

See merge request zdvresearch_bsc/adafs!70

Support basic directory functionalities

See merge request zdvresearch_bsc/adafs!38

Signed-off-by: Marc Vef <vef@uni-mainz.de>

In the glibc both readdir and closedir functions marks the @dirp parameter
with a non-null attribute.
If we use a compiler optimization level grater then 1,
the `dirp == nullptr` expression is evaluated to false
at compilation time and the relative if blocks is cutted out
of the final binary.

In order to avoid this bheaviour, we implement a functions and then set
that as weak alias of the real glibc function call.

readdir and closedir in glibc returns a EINVAL if the directory pointer
passed as parameter is NULL

Both __xstat and __lstat simply needs to invoke adafs_stat.