no need for list series if we're only looking for a single series

[Dieterbe]

we shouldn't execute queries like list series /^one_min\.perf_test\.test0\.metric8$/
if the pattern is only a single series, no need to do list series with a regex. there is probably something more efficient to check existence of a series. maybe a select limit 0 or desc <seriesname>
(or at least there should be)
cc @jvshahid @pauldix @vladimir-smirnov-sociomantic