Backport: relayd: replace lttng_index_file with relay_index_file

lttng_index_file is shared between the consumer and relay daemon.
However, the introduction of the fd-tracker in the relay daemon
makes it hard to cleanly share this piece of code between both
daemons.

The ctf-index.h header is still shared by both daemons which
is the most important part. The lttng/relay_index_file class
is a fairly thin wrapper around file system operations (unlink,
read, and write an index) so there is little value gained in
sharing the code vs heavily modifying it to handle the presence
of an fd-tracker in the process.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>