Fix: call lttng_poll_set_max_size before start threads

This changes the location of lttng_poll_set_max_size() call within
sessiond main() to call it before creating the ht_cleanup thread. This
fix is only needed on top of the main refactoring patch.

This fix also adds checks for lttng_poll_set_max_size() error values.

Fixes #747

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>