Refactor state dump system

Refactor state dump system

This patch refactors the state dump system. The state dump provider is
now "lttng_ust_statedump". Each process' state dump is now delimited
by a pair of "start" and "end" events. These events mark the beginning
and end of the state dump, which happens once per traced application
per session. Note that for a given (process, session) pair, begin/end
events are serialized and will match. However, in a session, state
dumps from different processes may be interleaved. The vpid context
should be used to identify which events belong to which process.

Signed-off-by: Antoine Busque <abusque@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>