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.

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/`

Daemon: root directory must be relative

See merge request zdvresearch_bsc/adafs!69

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

Daemon: introduce storage backend

See merge request zdvresearch_bsc/adafs!60

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.

Use CMake imported target for Boost components

See merge request zdvresearch_bsc/adafs!68

Introduce MetadataDB class

See merge request zdvresearch_bsc/adafs!67

Preload includes cleanup

See merge request zdvresearch_bsc/adafs!64

docker: changed dependencies build path

See merge request zdvresearch_bsc/adafs!66

The metadataDB class was using integer to return errors.
New exceptions classes has been introduced in order to be used as error
propagation method.

In this commit the MetadataDB::get function has been modified in order
to make use of exceptions.