bank.capitalization() can overflow

[rwalker-com]

Problem

bank.capitalization is a u64, which could overflow at some point

Proposed Solution

promote to a u128 and deal with bank deserialization versioning

[ryoqun]

Hehe, I and @svenski123 are working on this, this will be quite easy thing once all foundation (#10581) is merged like this: ryoqun@cd83759

[ryoqun]

I think this issue can be closed by #11927 because we're checking bank.capitalization are under u64::MAX periodically now.

Let's revisit this in distant future. :)