Make the format of sumdb.Server data tile responses compatible with those
served by sum.golang.org: Like formatted records for the lookup endpoint, but
without each record IDs.
Updates documentation for sumdb/tlog.FormatRecord about data tiles.
Server still calls FormatRecord to keep the validation, then removes the first
line.
For golang/go#69348
Change-Id: I1bea45b3343c58acc90982aaff5d41e32b06ae8c
Reviewed-on: https://go-review.googlesource.com/c/mod/+/618135
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>