Remove Timestamp::Now

[Empty2k12]

Description

Undoes #51

Due to inconsistencies between InfluxDb v1.7 and InfluxDb v1.8, I have decided to remove the Timestamp::Now value. Previously, when using this, InfluxDb would use the current system time in RFC3339.

The new way of using the current time for a write is to calculate the time in Rust and send it to InfluxDb along the request. This way, any delay on the way to InfluxDb is also not an issue any more and the time of creating the Query in Rust is the one being used.

One simple piece of code to achieve this is

use influxdb::{Timestamp};
use std::time::{SystemTime, UNIX_EPOCH};
let start = SystemTime::now();
let since_the_epoch = start
  .duration_since(UNIX_EPOCH)
  .expect("Time went backwards");
let query = Timestamp::Milliseconds(since_the_epoch)
    .into_query("weather")
    .add_field("temperature", 82);

The library could supply helper functions to calculate the timestamp to be used together with the existing Timestamp initializers.

Checklist

• Formatted code using cargo fmt --all
• Linted code using clippy cargo clippy --all-targets --all-features -- -D warnings
• Updated README.md using cargo readme > README.md
• Reviewed the diff. Did you leave any print statements or unnecessary comments?
• Any unfinished work that warrants a separate issue captured in an issue with a TODO code comment