internal/trace: add end-of-generation signal to trace

internal/trace: add end-of-generation signal to trace

This change takes the EvEndOfGeneration event and promotes it to a real
event that appears in the trace.

This allows the trace parser to unambiguously identify truncated traces
vs. broken traces. It also makes a lot of the logic around parsing
simpler, because there's no more batch spilling necessary.

Fixes #73904.

Change-Id: I37c359b32b6b5f894825aafc02921adeaacf2595
Reviewed-on: https://go-review.googlesource.com/c/go/+/693398
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>