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

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

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.