Percentile speculative execution

This policy has access to Metrics shared with session, and triggers speculative execution when the request to the current host is above a given percentile.

Example

To use this policy in Session:

#![allow(unused)]
fn main() {
extern crate scylla;
use scylla::Session;
use std::error::Error;
async fn check_only_compiles() -> Result<(), Box<dyn Error>> {
use std::{sync::Arc, time::Duration};
use scylla::{
    Session,
    SessionBuilder,
    speculative_execution::PercentileSpeculativeExecutionPolicy,
};

let policy = PercentileSpeculativeExecutionPolicy  {
    max_retry_count: 3,
    percentile: 99.0,
};

let session: Session = SessionBuilder::new()
    .known_node("127.0.0.1:9042")
    .speculative_execution(Arc::new(policy))
    .build()
    .await?;
Ok(())
}
}