Motivation:

Some OAI-PMH endpoints are broken; moreover, they're broken in such a
way that makes harvesting information wastes a lot of time without
producing useful information.

The specific example is the ISIS endpoint, which is both very slow (~10
seconds per request) and, after ~9 hours of havesting returns a
resumptionToken that results in failures in a subsequent ListIdentifiers
request.

The ESS endpoint is also broken.  While also annoying, the impact is
less because of special handling when a server hasn't provided useful
information.

The goal is to allow selective disabiling of harvesting while continuing
to update high-level OAI-PMH information based on the Identity call.

Modification:

Add a `skip-harvesting` boolean option.  If set with the value true then
harvesting is skipped for this endpoint.

Result:

It's possible to update all endpoints without a very long and fruitless
time spent harvesting from broken endpoints.