Support backoff retry strategy in Retry.go

[isopropylcyanide]

Currently, tchannel-go/retry.go offers RunWithRetry(runCtx context.Context, f RetriableFunc) which is a straight for loop of the func upto retryOpts.MaxAttempts.

It does not offer a way for callers to specify a backoff strategy.

A backoff strategy (such as backoff.Exponential or backoff.FixedDelay) would be useful to not wreak havoc at an unhealthy backend. The current implementation will shoot 1 + 5 (default retries) calls at the backend.

The proposal is to support a backoff strategy in RetryOptions

// RetryOptions are the retry options used to configure RunWithRetry.
type RetryOptions struct {
	...
	TimeoutPerAttempt time.Duration

        BackoffStrategy BackoffStrategy
}

var defaultRetryOptions = &RetryOptions{
	MaxAttempts: 5,
        BackoffStrategy : nil // or fixed or backoff
}

• If BackoffStrategy is nil, the existing behaviour continues.
• If BackoffStrategy is fixed delay, implement a fixed delay between retries
• If BackoffStrategy is exponential, implement an exponentially decaying delay exponentially between retries.