There was a subtle and nasty bug in listSeriesIterator.Seek.
In addition, the Seek call is defined to be a no-op if the current
position of the iterator is already pointing to a suitable
sample. This commit adds fast paths for this case to several
potentially expensive Seek calls.
Another bug was in concreteSeriesIterator.Seek. It always searched the
whole series and not from the current position of the iterator.
Signed-off-by: beorn7 <beorn@grafana.com>