Set datakey for first table in stream writer

[jarifibrahim]

If encryption is enabled on a DB and stream writer is used then the
first file SST would not be encrypted, only the SSTs after the first one
would be encrypted. This PR fixes it.

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This change is 

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[coveralls]

Coverage increased (+0.6%) to 78.471% when pulling 9bf1c0b on ibrahim/streamwriter-encryption into a425b0e on master.

[coveralls]

Coverage decreased (-0.2%) to 77.691% when pulling 5608256 on ibrahim/streamwriter-encryption into a425b0e on master.

[jarifibrahim]

Reviewable status: 0 of 2 files reviewed, 3 unresolved discussions (waiting on @ashish-goswami, @golangcibot, and @manishrjain)

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stream_writer.go, line 128 at r2 (raw file):

	}

	for streamID, req := range streamReqs {

This was changed to make golint happy. See #1054 (comment)

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stream_writer.go, line 216 at r2 (raw file):

}

func (sw *StreamWriter) newWriter(streamID uint32) (*sortedWriter, error) {

This was changed to make golint happy. See #1054 (comment)

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[ashish-goswami]

Reviewable status: 0 of 2 files reviewed, 4 unresolved discussions (waiting on @ashish-goswami, @golangcibot, @jarifibrahim, and @manishrjain)

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stream_writer.go, line 134 at r2 (raw file):

			writer, err = sw.newWriter(streamID)
			if err != nil {
				return errors.Wrapf(err, "failed to create writer with ID %d", streamID)

I think error can be wrapped only once.

[jarifibrahim]

Reviewable status: 0 of 2 files reviewed, 4 unresolved discussions (waiting on @ashish-goswami, @golangcibot, and @manishrjain)

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stream_writer.go, line 134 at r2 (raw file):

Previously, ashish-goswami (Ashish Goswami) wrote…

I think error can be wrapped only once.

Done. See newWriter method. It no longer wraps the error.

[jarifibrahim]

Merging this PR since it's a simple change and doesn't require @manishrjain's review.