Cleanup: src.ctf.lttng-live: move function declarations around

Cleanup: src.ctf.lttng-live: move function declarations around

* Remove declarations of `lttng_live_remove_stream_iterator()` and
`lttng_live_add_stream_iterator()` functions as they are not used.

* Move `lttng_live_need_new_streams()` function to viewer-connection.c
as it's only used in this file.

* Move declaration of `lttng_live_create_viewer_session()` to
viewer-connection.h

* Namespace `lttng_live_session` related functions with the
`lttng_live_session_` prefix.

* Rename `lttng_live_borrow_trace()` function to
`lttng_live_session_borrow_or_create_trace_by_id()`

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: Ie18423e9a39b4ac1e04642f381a9f59831c34abe
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2386
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>