Allow specifying nanosecond durations

The current version of influxql.ParseDuration used by Kapacitor doesn't support parsing nanosecond durations. Now that influxql is its own package it might be easier to update to the latest version.

Specifying nanosecond durations is useful when converting integers to durations. See #1648

Start by inspecting influxql.ParseDuration and how Kapacitor uses the influxql package. Review issue #1648 for the integer-to-duration context, then verify that nanosecond durations parse successfully and add or update coverage for that behavior.

Written by the indexing model from the issue text.