Tweak to active total migrations

[gavofyork]

One extra migration, in case there are multiple accounts with inactive funds.

[kianenigma]

I think the many variant is sufficient for all cases, as you can always provide a single element Vec.

[KiChjang]

A reminder that we need to add the MigrateToTrackInactive migration struct to the runtimes after this is done.

That does however bring up a question: which migration struct should each runtime use? MigrateToTrackInactive or MigrateManyToTrackInactive? As it is written, each runtime can only use one or the other, but not both at the same time.

[gavofyork]

Right. We will only need the first one for all chains we use, however I cannot rule out the possibility that ecosystem chains have multiple accounts which contain inactive funds so the second variant is provided.

Of course the second is strictly more general, however using it would require additional verbosity since the Checking Account is always stored in a single already-exposed type param which fits neatly into the first variant and not the second. So it's provided in the interest of easing developability and reducing the chance of bugs creeping in from tedious boilerplate.