[HEAD MERGE] Implement create session snapshot API in lttng-sessiond

Signed-off-by: David Goulet <dgoulet@efficios.com>
[HEAD MERGE] Implement lttng create --snapshot command

Signed-off-by: David Goulet <dgoulet@efficios.com>
Fix: snapshot support for UST and kernel in same session

Signed-off-by: David Goulet <dgoulet@efficios.com>