Fix: take index seq number into account for rotation pending check

The rotation pending check is only performed on the sequence number of
the received data. However, it is expected that the index of the
stream has been written to disk by the time this check returns that
no rotation is pending.

This patch ensures that the minimum between the data and index
sequence numbers are used to perform this check.

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