Make the MAX_SERIES limit a plugin parameter.

[StephenSorriaux]

Add a new plugin parameter to change the maximum number of series that will be passed to the frontend.

The current value of 120 is kept for:

• backward compatibility (after the plugin update, no value will be set, so 120 will still be used);
• incorrect value set in the configuration panel (only round positive number are correct values).

Fixes https://github.com/grafana/influxdb-flux-datasource/issues/31

[StephenSorriaux]

Hey,

Any feedback about this pull request?

Thanks

[ryantxu]

In the next release, this is supported on the serverside and has a configuraiton: https://github.com/grafana/influxdb-flux-datasource/blob/master/src/components/ConfigEditor.tsx#L92