Add support for rate limits

[whymarrh]

Per Infura Documentation § Rate Limits:

Infura accounts will be subject to rate limits after exceeding their daily request limit. We will also rate limit projects that send too many JSONRPC requests during a short time window. These rate limits are in place to ensure the reliability of our service for everyone using it.

If you are rate limited, your JSONRPC responses will have HTTP Status code 429 and contain a JSONRPC error response [...]

The "data" section contains 3 fields about your rate limit:

• current_rps: The current rate per second determined by Infura
• allowed_rps: The current allowed rate which you should stay under.
• backoff_seconds: The suggested amount of time to wait before sending more requests.

This module currently supports the idea of requests that can be retried, and could also support the idea of rate limits:

1. Hitting the rate limit should wait backoff_seconds before retrying a request; and
2. Once an allowed_rps is received the request rate should be adjusted to stay under that value.

[Gudahtt]

Updated documentation link: https://docs.infura.io/networks/ethereum/how-to/avoid-rate-limiting