Automatically retry requests when appropriate

[Castaglia]

The mod_aws module currently does not implement any retry functionality. For better resiliency, mod_aws should retry requests on the following conditions:

• 5xx HTTP response codes (vs 4xx response codes)
• server errors/issues (vs client/sender errors/issues)
• RequestLimitExceeded error code
• ECONNRESET errros

See the AWS retry docs for more details.

Note that all retries should happen using exponential backoffs with jitter.