types: add Time3339.String and fix bug to use RFC3339Nano

Change-Id: Ib86b7ea86d9afe7738fc9a3ee9fa18fd2abef3b1
schema: use better types in superset: blobref and Time3339

Change-Id: I7004220789e1048c1ebcc8703bf2948a733bae10
search: split metaMap into metaMap and metaMapThumbs

Change-Id: I5a29166be7ed44a7c6a954ba689f1f3dbfbe57d7