sessiond: transition from lttng-ust to tracer agnostic API

Refactor the session daemon's user space tracer management to use the
tracer-agnostic trace hierarchy description API. Since the API
introduced under lttng::sessiond::trace is closer to idiomatic C++,
some changes are needed to make use of it.

The biggest changes make the ust_registry* structures inherit from the
trace descriptions classes (trace_class, stream_class, event_class,
clock_class). This effectively isolates the members of the
`ust_registry` structures that describe their corresponding CTF class
to a base class and leaves only the implementation guts in the
`ust_registry` objects (moved under lttng::sessiond::ust).

Since the generation of TSDL metadata directly used the lttng_ust_ctl
API, it is replaced by a new implementation that is based around a trace
class visitor that serializes the tracer agnostic trace description
objects.

The TSDL environment visitor is moved under the TSDL implementation
of the trace class visitor.

Some little changes are also made to the existing code to make it
exception-safe, as needed.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I6562ef12e85afa91780337e8c6fdbb13568d9ece