relayd: keep track of prev_index_seq in relayd_stream

The rotation and data pending checks are only performed on the
sequence number of the received data. However, it is expected
that the index of the stream (when applicable) has been written
to disk when those checks say that their respective operations
have completed.

This patch only introduces a new 'prev_index_seq' position that
is updated when an index is flushed to disk.

A follow-up fix addresses the issue mentioned above.

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