relayd: open live viewer files from the current stream's trace chunk

Since relay_streams' paths are now relative to a session's trace
chunk, file operations performed by the live protocol handler must
occur through a trace chunk.

Note that viewer streams hold a reference to the current trace chunk
of their corresponding relay stream. This is fine right now as no
session rotations may occur during the lifetime of a live session. If
this was allowed, this would be a problem since a session rotation
would not complete until all live viewer streams release their
reference to the trace chunk.

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