Add snapshot mode to lttng list session

Signed-off-by: David Goulet <dgoulet@efficios.com>
Bump UST ABI major version for 2.3 release

Signed-off-by: David Goulet <dgoulet@efficios.com>
New UST default buffers is now per UID

Signed-off-by: David Goulet <dgoulet@efficios.com>
[MERGE] fix per UID default buffer

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