Add more anti-spam mechanisms

[metaproph3t]

The main attack vector I can think of at the moment is spam. Someone could create 100 proposals, for example, and then the honest participants would be playing whack-a-mole against the attacker where the attacker would manipulate a market, causing people to join that market, then move to another market, and so on.

Two solutions I can think of:

• Use an exponential curve for the SOL fee, so that it's a lot right after a proposal was created but it asymptotes around 0.5 days mark
• Require that the conditional vault for the proposal hold at least x% of the META supply