internal/worker: load shed based on DB lock contention

If many DB processes are waiting for locks, shed load.

This is in addition to the existing load-shedding rule based on zip
size.

I conducted an informal experiment to see how this worked. I queued up
82 versions of github.com/aws/aws-sdk-go for processing. That module
has a small zip, so no shedding occurs because of zip size, but it
takes some time to process.

Without lock-based load-shedding, there were 157 "max serialization"
errors. Most of the time there were many active fetches in progress,
almost all waiting for locks.

With lock-based load-shedding, there were only 48 "max serialization"
errors. Most fetches completed quickly.

For golang/go#48010

Change-Id: I0cee02b9c4085a8bc187d803eaca2f30ddd378b5
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/349312
Trust: Jonathan Amsterdam <jba@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Jamal Carvalho <jamal@golang.org>
Reviewed-by: Julie Qiu <julie@golang.org>

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