sessiond: generate packet header, packet context and event header dynamically

sessiond: generate packet header, packet context and event header dynamically

Instead of hardcoding event header and packet header/context layouts,
their layout is expressed using the lttng::sessiond::trace::field/type
API and serialized to TSDL using the visitor.

This reduces the duplication of code in the CTF2 visitor.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I9a4255889e36e6861a59536ce70b1e708da06ac5