In the current implementation, it is possible for a client to
continuously send warning alerts, which are just dropped on the floor
inside readRecord.

This can enable scenarios in where someone can try to continuously
send warning alerts to the server just to keep it busy.

This CL implements a simple counter that triggers an error if
we hit the warning alert limit.

Fixes #22543

Change-Id: Ief0ca10308cf5a4dea21a5a67d3e8f6501912da6
Reviewed-on: https://go-review.googlesource.com/75750
Reviewed-by: Adam Langley <agl@golang.org>
Reviewed-by: Filippo Valsorda <hi@filippo.io>
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>