Fix s3 upload performance regression

[pracucci]

The symptom

The PR #66 introduced a regression in the S3 upload performance. After that commit, S3 uploaded are significantly slower. Here you can see how the upload latency changed for one of our Grafana Mimir clusters running in AWS:

Change deployed to prod on 10/06, where the latency increase. Since the upload performance in Mimir is not critical (e.g. doesn't affect neither read and write path latency), it was undetected for quite some time. Until today.

## The cause

Why the regression? Minio has some optimizations when the reader implements io.ReaderAt because it can leverage on .ReadAt(). We're used to call Upload() with os.File as a reader (I think Thanos does the same). After #66 wrapped the reader with the timing reader, which doesn't implement io.ReaderAt and so Minio couldn't optimize the upload anymore.

The fix

In this PR I propose a fix to the issue, introducing timingReaderSeekerReaderAt which embeds timingReaderSeeker, which in turn embeds timingReader.

To simplify the code, I've removed the usage of nopSeekerCloserWithSize() and NopCloserWithSize() in the Upload and pushed down to timingReader whether we want it to Close() the wrapped reader or not. The renaming from timingReadCloser to timingReader follows this change, given it takes in input io.Reader.

Manual test

I've manually that this PR fixes the upload speed regression, running a test in AWS which uploads 1GB object to S3 multiple times.

Before this PR

S3 bucket not wrapped with metrics:

With metrics: false Elapsed: 2.262466521s
With metrics: false Elapsed: 2.328912616s
With metrics: false Elapsed: 2.312762143s

S3 bucket wrapped with metrics (see higher latency):

With metrics: true Elapsed: 9.590695888s
With metrics: true Elapsed: 8.402357109s
With metrics: true Elapsed: 7.858041235s

After this PR

S3 bucket not wrapped with metrics:

With metrics: false Elapsed: 2.767424849s
With metrics: false Elapsed: 2.066829353s
With metrics: false Elapsed: 2.114329674s

S3 bucket wrapped with metrics (see no higher latency):

With metrics: true Elapsed: 2.728111635s
With metrics: true Elapsed: 2.216537896s
With metrics: true Elapsed: 2.392596594s

[MichaHoffmann]

lgtm