Rent can be easily avoided with short-lived rent exempt accounts

[jstarry]

Problem

Creating rent exempt accounts is pretty cheap right now (~0.0071 SOL per KB on Mainnet Beta) so it's trivial to create them en masse. No matter when these accounts are fully withdrawn (same epoch, 2 epochs later, etc), they do not need to pay any rent. They will only ever pay rent if their balance falls below the rent exempt threshold. This is all in-line with our rent design but this feels like an economics incentive bug because there's very little reason to ever pay rent.

Proposed Solution

A few potential fixes:

1. Increase rent fees
2. Impose a lockup on rent-exempt accounts (min 1 epoch?)
3. Charge retroactive rent if withdrawn early (within 1 epoch?)

Proposed solutions (2) and (3) aim to enforce the "reduced liquidity" aspect of the rent economic design

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EDIT: Actually ~7 SOL per MB doesn't seem so cheap. Maybe this is fine as is.

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