- improved statfs system call
 - intercepted statvfs library call

Both of the above function now report the aggregated real size
of all the data-nodes.

You can test this with `df "/tmp/gekkofs_mountpoint"` command

Metadata class needs to be used by both the client and the daemon

The metadata class needs to represent metadata of a file,
thus it shouldn't contain the path itself. In fact we actually didn't
use it at all.

This will make further refactoring easier and it save some memory copies.

The decrement size operand does not have a trailing slash, thus we need
to add it before to use std::stoul.

Implemented all the logic to handle truncate operation.

Test: added truncate test

According to the read manpage a read could return less bytes then
requested due to an interrupt or an error.

We now try to repeat the read operation until all the existent data have
been read correctly.

The ChunkStorage class now store the chunksize in order to perform
runtime checks upon chunks bundaries.

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

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

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.

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.