Move UST registry into sessiond and implement notifiers

Notify now comes from the UST tracer to register event in a new registry
in the session daemon storing UST channel and events. This registry is
used to send back event and channel IDs to the tracer and the session
UUID. It will also be used later on to generate metadata which will
completely remove metadata generation from the application.

This introduces a new thread in the session daemon being the thread
managing application notification using a new socket (notify socket).
This thread is in ust-thread.c/.h and spawned at startup.

Needs to be use in locked-step with LTTng-UST commit:
"Move UST registry into sessiond and implement notifiers"

Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>