hashtable: replace non-const iterator node accessors by a const version

hashtable: replace non-const iterator node accessors by a const version

The various lttng_ht_iter_get_node_* functions are not const-correct which makes
their use difficult in some places. This is mostly due to the fact that
cds_lfht_iter_get_node (from liburcu) is, itself, not const-correct.

These functions are replaced by a single templated function that is
const-correct by virtue of inlining the trivial iterator node accessor of
liburcu.

Change-Id: I4df87abedb1ec6b14eb52ce4c212a10805898954
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>