Initialize relay_stream chunk_id to its session's current trace archive id

Initializing the relayd's streams with a stream_chunk_id allows the
relayd to differentiate between a stream created before the first
rotation (at chunk id == 0) vs. a stream that has been created
after the last (or pending) rotation.

Before this fix, the relayd can fail to identify that a rotation
has been completed.

This is caused by the fact that a stream's chunk id is initialized
to 0 and updated by the RELAYD_ROTATE_STREAM command to the
id of the chunk that is currently being rotated.

The 'stream->current_chunk_id.value < chunk_id' check performed
by the RELAYD_ROTATE_PENDING will cause rotations to never
complete for streams that are created between the launch of a
rotation and the check for its completion.

For example, when the relayd is checking whether the rotation id
'3' is completed, it may see streams with the default value of
their chunk id set to '0' and determine that a rotation is still
pending.

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