Add testpoints in lttng-sessiond for each threads

Add testpoints in lttng-sessiond for each threads

This commit adds 8 new testpoints in the lttng-sessiond binary. These
testpoints rely on the testpoints infrastructure introduced recently.

Testpoints:

thread_manage_clients
thread_manage_clients_before_loop
thread_registration_apps
thread_manage_apps
thread_manage_apps_before_loop
thread_manage_kernel
thread_manage_kernel_before_loop
thread_manage_consumer

The thread_<thread_name> testpoints are placed directly at the thread
start and they can be used to trigger failure in <thread_name>.

The thread_<thread_name>_before_loop testpoints are placed directly
before the main processing loop of the thread and thus can be used to
stall the processing of the thread.

Signed-off-by: Christian Babeux <christian.babeux@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>