ctf: allocate some structures with new

ctf: allocate some structures with new

Allocate some key structures in the ctf plugin with new (and free with
delete), so we can start using non-POD fields in them.  Initialize all
fields to 0 / nullptr to replicate what g_new0 did.

Add ctf_fs_ds_index_entry_destroy, to abstract how this object is
destroyed (it is currently being g_free'd at multiple places) and to use
it as a g_ptr_array_new_with_free_func callback.

Change-Id: Ic0f893655db22c964641639aa0625d820af71587
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8021
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12195
Tested-by: jenkins <jenkins@lttng.org>