When deep copying Sam/Variant records, do not also deep copy the header

Deep copying the headers every time a record is copied is wasteful and
unnecessary, given that headers are immutable (with the single exception
of VariantHeader::advanced_merge_header(), which should be moved to a
builder)
Merge pull request #124 from broadinstitute/dr_remove_header_deep_copying

When deep copying Sam/Variant records, do not also deep copy the header