Fix multiple error handling for UST tracing

Bad error handling was making event and channel out of sync between
trace ust and ust app data structure.

This makes the session daemon way more stable for the case the sessiond
and/or UST tracer reach the maximum number of open files.

Fix memory leaks upon error when creating a session/channel/event.

Fix out of order add unique channel node to hash table.

Finally, fix some debug/error statements.

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