[ADAM-353] Fixing issue with SAM/BAM/VCF header attachment when running distributed

Fixes issue #353. Runs header attachment per partition; not the cleanest fix, but it'll do. Modifies the SAMHeaderWritable class; without this modification, I was running into serialization issues.

@ryan-williams Can you give this a shot on your end? I've tried this on EC2 and it seems to work OK but would like to confirm that it works OK for you before merging it.
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