Influxdb-java data model should have Series

In current influxdb-java data model we find all concepts of InfluxDB except Series (refer to https://docs.influxdata.com/influxdb/v1.6/concepts/key_concepts/).

Series is the core concept in any time series database. A time series DB design suggestion is [don't have too many series](https://docs.influxdata.com/influxdb/v1.6/concepts/schema_and_data_layout/#don-t-have-too-many-series), that means most of data points would belong to a certain series.

Nevertheless, a certain time data source (such as a weather censor, a surveillance camera ...), in my opinion, will be writing to a specific series only.

So it would be more convenient for user if we can have a Series in influxdb-java, and also fulfill the InfluxDB model from high level POV

One design I would like to propose is as follows


In this design, SeriesPoint extends Point and refers to Series, so all SeriesPoints belong to a Series can share the same tag set and retention policy. This way we can even save some overheads when generating line protocol string