Need to be able to hold enough banks in memory to survive a multi-hour network outage

[mvines]

At ~5MB of RAM per Bank and a 2.5 Banks a second, 32GB of RAM can be filled with unrooted Banks in about an hour. This is not long enough to survive a temporary networking problem.

Really we need to be able to an Epoch worth of unrooted Banks (after which point it doesn't matter because we run out of leader schedule).

[ryoqun]

@mvines @carllin Could this be alleviated by this? #8958

[mvines]

Not sure, I think we just need to test it (as probably a nightly/weekly regression test) and see how we do