Ignore HTTP Keep-alives

[lawrencegripper]

As per the change in Cockroach leaktest I'd like to ignore HTTP keep-alives in this utility.

leaktest: ignore HTTP keep alives
Tests that performed an HTTP connection using the default client had to
have a special instruction to poke http.DefaultClient to disable keep
alives to prevent the leaktester from failing. An upcoming change performs
an HTTP connection with its own unconfigurable client, which would have
required a bunch of special code just to make the leak tester pass.
Instead, ignore stacks that are included during a connection in keep
alive. This matches the code in net/http/main_test.go#afterTest.

cockroachdb/cockroach@e5e6dde#diff-fdc7f75e12e2453349457c6742ff532f