This patch reflects eventual allocation errors in a more specific way to
the caller of 'alloc_aligned', in particular out-of-metadata and
out-of-memory are considered as different conditions.
Related to issue #526.
Revert the core-local mapping created in 'Ram_session_component::_clear_ds()'
and free the virtual memory region allocated for this mapping when a
RAM dataspace gets freed.
Fixes#416.
