Fix: validate whether event notifiers are supported by UST application

Fix: validate whether event notifiers are supported by UST application

Considering that lttng-sessiond interacts with older (2.12) applications
as well as 2.13+, it needs not to send event notifiers related commands
to an older UST, because the unknown command will cause the protocol to
become out of sync on the communication socket, because the older
lttng-ust does not know how many bytes must be read when receiving the
unknown command. So even though it can return that the command is
unknown, the communication socket becomes out of sync.

Depends-on: lttng-ust: If048c739dd37147ffb2a54715c2101177d2df4f7
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: If9cbc7a7cd6cb957134bf3a2e284ab99ef20e93b