SetRetryCount allows negative values that Backoff doesn't properly handle

[moorereason]

SetRetryCount allows values less than zero, but the Backoff implementation assumes opts.maxRetries is zero or greater. The result is that Backoff doesn't actually make a request but also doesn't return an error.

Two options for a solution:

1. Convert negative values to zero in SetRetryCount.
2. Convert a negative value into math.MaxInt64 (retry forever) in SetRetryCount.

In either case, Backoff should be updated to return an error if opts.maxRetries is less than zero.

Related to #392

[moorereason]

@yjhmelody commented here that option 2 (retry forever) is preferred.

I tend to agree. Normally, a -1 in this scenario means "no limit."

[jeevatkm]

@moorereason @yjhmelody Just wondering, the forever retry option is good or bad for the client library since it will result in DoS behavior on targeted application/service.

[jeevatkm]

Prevent -1 usage with error message