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

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

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.

Support for the following calls has been added:
 - opendir
 - readdir
 - closedir

The readdir call use the new RPC get_dirents that ask to all the
nodes (broadcast) about all the existing first-level entries
of a specific directory.

In order to support directories functionalities
opendir,readdir,closedir,etc.. the OpenDir class has been introduced.
This is a new specialization of the OpenFile class so that OpenDir
object can be stored in the open_file_map along with regular file.

Allow to add already instantiated OpenFile to the OpenFileMap.

This will come in hands when we will have sub-classes of the OpenFile.
In that case this new add function will accept all object that are
derived from the OpenFile class. See directories functionalities.

exposes a new  DB function that allows to retrieve all the
first-level entries of a specific directory.

In the case the open call has been called with the O_DIRECTORY flag the
ENOTSUP (Operation not supported) will be returned.

The path relativization code was not taking into a account corner cases
such as:
 - `/tmp/mountdir`
 - `/tmp/mountidr/`

Now that we use only relative path on daemon side,
also the initial creation of the root directory should be
use a relative path.

Acknowledge the main thread that the IO operation had terminate before
to actually close the file.

This is a "dirty" fix that waits to be refactored.

WARNING: Error on close will not be reported to the client that issued the IO
operation.

Data operation has been grouped under a ChunkStorage class
that will manage all the data backend operations.

The instance of this object is stored into the ADAFS_DATA sigleton.