keep CompleteMultipartUpload alive

[lperlaki]

reopen of #31

changes the response body from the CompleteMultipartUpload Operation to a custom KeepAliveBody that sends a whitespace every 100 milliseconds to keep the request alive

but now with hyper support for trailers

fixes #30

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[Nugine]

We have a new failure in e2e tests. Maybe a compatibility issue?

{
  "name": "aws-sdk-php",
  "function": "uploadPart ( array $params = [] )",
  "args": {
    "Bucket": "aws-sdk-php-65f762b387bb0",
    "Object": "obj1"
  },
  "duration": "261",
  "status": "FAIL",
  "error": "Your proposed upload is smaller than the minimum allowed object size."
}

[Nugine]

I'm busy for the next three weeks. I'll look into the e2e test failure after that. Sorry about the delay but finally we will complete.

[Nugine]

It seems that we hit a bug in aws-sdk-php.

• S3: Forward parsed error context in AmbiguousSuccessParser.php aws/aws-sdk-php#2932

I'll allow the failed test and merge this PR soon.