correctly order condition check to reduce # of scans for delete api (https://rbcommons.com/s/pepperdata/r/5442)

noticed during additional gagg testing that tsd delete scan row count was higher
than delete row count by a noticeable margin. turns out the row limit breaking
condition was checked too soon, resulting in an extra scan. doesn't affect
delete functionality but causes some wasted cycles.

Submit on behalf of George to release OpenTSDB. The original CL is
https://rbcommons.com/s/pepperdata/r/5435/.