Add consumer-stream.c/.h in libconsumer

These new files contain relevant calls to a lttng_consumer_stream object
and for now breaks down consumer_del_stream() from consumer.c that got
out of hand in terms of size and can not be used if the stream was not
previously successfully sent to a thread making difficult the deletion
of a stream in some error path or in no monitor mode (future mode for
streams with snapshots) outside of this use case.

It helps also modularize the consumer object and makes the code easier
to maintain and to better evolve.

At this commit, no components use this yet. Future work will
incrementaly move every consumer stream specific actions to that
seperate API.

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