keep import position in whisper reader

[replay]

Tested in my test env. I ran the importer-reader with the following command:

./mt-whisper-importer-reader \
-dst-schemas /home/mst/documents/code/go/src/github.com/grafana/metrictank/scripts/config/storage-schemas.conf \
-http-endpoint http://localhost:5432/chunks \
-name-prefix my.test. \
-orgid 1 \
-threads 3 \
-verbose \
-whisper-directory ziggurat  \
-position-file /tmp/mypos

After a few seconds I aborted with Ctrl+C. The file /tmp/mypos now contains the list of files that have already been processed:

mst@mst-nb1:~/documents/code/go/src/github.com/grafana/metrictank$ cat /tmp/mypos
ziggurat/GenericJMX/cassandra_ClientRequest/counter/RangeSliceLatency_count.wsp
ziggurat/GenericJMX/cassandra_ClientRequest/counter/RangeSliceFailures_count.wsp
ziggurat/GenericJMX/cassandra_ClientRequest/counter/RangeSliceLatency_sum.wsp

When I now restart the same command it skips those that have already been completed:

DEBU[0000] Skipping file ziggurat/GenericJMX/cassandra_ClientRequest/counter/RangeSliceFailures_count.wsp because it was listed as already done 
DEBU[0000] Skipping file ziggurat/GenericJMX/cassandra_ClientRequest/counter/RangeSliceLatency_count.wsp because it was listed as already done 
DEBU[0000] Skipping file ziggurat/GenericJMX/cassandra_ClientRequest/counter/RangeSliceLatency_sum.wsp because it was listed as already done 

fixes issue #727

[Dieterbe]

i notice that you mix the position"tracker" terminology with position"keeper"