[HEAD MERGE] Implement the relayd live features

Create relayd session from the sessiond with additional parameters
Relayd listen on 1 additionnal port
Version exchange with the viewer
List the sessions with number of connected client (max 1 for now)
Attach to a session with seek parameter (begin or last for now)
List the streams of a session
Get the indexes per stream
Get data packets
Flag metadata streams
Match ctf_traces based on the trace path
Handle the synchronisation of the metadata (prevent the viewer from
reading a packet if it does not have the complete metadata)
Send the metadata
Ioctl to get the current timestamp
Inactive stream beacon

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
[HEAD MERGE] REVIEW CAREFULLY : test for new metadata at each packet

After sending each data packet in live, we need to check if new metadata
is available before sending the index informing the viewer it can read
the trace.
Since the data and the metadata are handled by two different threads,
this patch introduces a rendez-vous point : if new metadata is
available, the data thread flushes the metadata stream and waits on a
conditionnal variable. When the metadata thread finishes to send its
data, it wakes up the data thread which can send its index.

That way, the viewer is informed new metadata is available before
attempting to read a packet that might require an update of the
metadata.

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
[HEAD MERGE] relicense lttng-index and lttng-viewer to BSD

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>