Sync variant headers in VariantHeaderBuilder when building headers

htslib no longer calls bcf_hdr_sync() for us each time we modify
a variant header, so we need to call it ourselves in VariantHeaderBuilder
at build time
Sync variant headers when merging multiple headers

Another consequence of htslib's new, lazier approach to variant
header synchronization: we need to explicitly call bcf_hdr_sync()
when merging variant headers