set rent_epoch to Slot::MAX for rent exempt accounts

[jeffwashington]

Problem

#26509 got activated on mnb.
we can no longer create rent paying accounts.
rent_epoch will become meaningless once there are no longer rent paying accounts.
#26509 causes us to 'freeze' rent_epoch where it is when rewrites occur.
It would be helpful to freeze rent_epoch at a known sentinel value which is consistent for all rent exempt accounts.
Slot::MAX is such a sentinel value.
0 is already used at various times, but we could re-use it. There will be complications because we have special case code for 0 right now.
Before we stop doing rewrites (which is 1.15 at best), we need to have stabilized on the final value for rent_epoch for rent exempt accounts. Since each account's hash is dependent on rent_epoch, and we hope to stop storing account hash values in append vecs, we need the hash to be stabilized, using this constant, frozen value.

Also, once almost all accounts have a rent_epoch field of the same value, we can ultimately eliminate this field from append vecs and thus, snapshots, in-memory mmaps, the write cache, and all in-memory representations of an account. This is a step towards being able to remove this field completely.

Proposed Solution

[jeffwashington]

#28683