[feature] Retries

[carlcortright]

Support retries with a fixed interval backoff.

[onrik]

Why milliseconds? Golang has the type time.Duration

[onrik]

Thanks for PR. But i think this is not responsibility of ethrpc. Retries can be implemented by transaport in custom http client.

package main

import (
    "fmt"
    "log"
    "time"

    "github.com/onrik/ethrpc"
)

type RetryTransport struct {
	transport http.RoundTripper
	attempts  int
	delay     time.Duration
}

func (t *RetryTransport) RoundTrip(request *http.Request) (r *http.Response, err error) {
	for i := 0; i < t.attempts; i++ {
		r, err = t.transport.RoundTrip(request)
		if err != nil {
			time.Sleep(t.delay)
			continue
		}
	}

	return
}

func main() {
    client := ethrpc.New("http://127.0.0.1:8545", WithHttpClient(&http.Client{
       Transport: &RetryTransport{
		transport: http.DefaultTransport,
		attempts:  10,
                delay:     10 * time.Millisecond,
       }
    })
}