Add data structure for the data available command

The new stream hash table indexed by session id is added. The per
consumer stream mutex used for the synchronization between threads
accessing the stream which will be indexed in two hash tables.

Also, the data available command is added to both UST and kernel
consumer but for now returns ENOSYS. Just set the skeleton for the
implementation.

Finally, the session id is added to the trace-kernel (where it already
exists for UST) so it could be used later on for the data available
command.

This session id is also added in the consumer data structure for the
same purpose.

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