Fix: sessiond: tsdl: don't prepend underscore for stream_id

Fix: sessiond: tsdl: don't prepend underscore for stream_id

Although the CTF v1.8 specification recommends ignoring any leading
underscore, Some readers, such as Babeltrace 1.x, expect special
identifiers without a prepended underscore.

This causes problems in a follow-up patch since packet header, context,
and event headers become dynamically generated.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I0aeb24ab2e1c0ee1ea70d2fb9bb0eabbb2e9e035