src.ctf.fs: emit stream activity beginning/end messages

Emit the newly introduced stream activity beginning/end messages from
`src.ctf.fs`.

With CTF 1.8, there is no way to know the true beginning and end times
of a stream's activity. We cannot rely on the first packet's beginning
time and the last packet's end time because this is not what they mean:
they are, in fact, used to find the time ranges of the discarded events
and packets.

I'm adding (internal) bt_msg_iter_set_emit_stream_beginning_message()
and bt_msg_iter_set_emit_stream_end_message() which control whether or
not the stream and stream activity beginning messages, and the stream
and stream activity end messages are emitted. This is used to support
more easily and efficiently multiple files per data stream: beginning
messages are enabled for the first stream file and end messages are
enabled for the last stream file.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>