I find excluding -999.0 values annoying in practice.

example:

```
let a: Option<d128> = Some(d128::from_str("123.456").unwrap());
let b: Option<d128> = None;
measure!(influx, meas, D(a), D(b));
```

in the `None` case, no field is added at all.

- adds example to repeatedly send many measurements to InfluxWriter for purposes of stress testing (i.e. shut influxdb off, see if InfluxWriter recovers correctly)
- InfluxWriter now tracks allocated buffer memory and logs it
- lower INITIAL_BACKLOG

allows optional compression on rotated logfiles