UST periodical metadata flush

Add a socket between the sessiond and the ust-consumer to allow
periodical flush of the metadata channel.

If enabled (by specifying the --switch-timer option on the metadata
channel), a new timer thread in the consumer asks the session daemon for
new metadata for a specific session.

All the metadata collected is written into a metadata cache in the
consumer, this mechanism is useful for synchronisation (to avoid race
conditions between two metadata updates) and will also be useful when we
implement the snapshots.

Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>