Avoid modifying global state of other packages (dask) at import

[pelson]

It is bad form to modify a package's global state when another package is imported.

We shouldn't be changing the dask options when iris._lazy_data is imported. If it is that important to us, we should have our own state that we modify and maintain.

Note: Cartopy does a similar thing, and it is bad news.

[pelson]

Discussion with @dkillick. Pseudo code of one potential approach:

iris.config.get

    if iris.config._da_getter is explicitly set
        use it
    else
        get da.config
            if the bad one
                change to the default iris one
            else
                use da.config

[pelson]

Closed by #2879