Update variants.avdl
Update variants.avdl

Cleanups to align with new variants.rst overview, and to restore a few substantive changes from master (e.g. timestamps).
heuermh/ga4gh-schemas,ga4gh/ga4gh-schemas,diekhans/ga4gh-schemas,saupchurch/schemas,ga4gh/schemas,pcingola/schemas,dcolligan/schemas,diekhans/ga4gh-schemas,macieksmuga/schemas