handle growing number of user sub accounts

[0xbigz]

the problem:
the protocol has max sub account limits to keep performance high for existing accounts and avoid any sort of performance based attack. for instance, i've been told a larger number subaccounts create too much infrastructure strain (rpc, history tracking, etc). The temporary account rent on solana is not sufficiently high to make sense for the dapp's economics here.

potential solutions:

• more active garbage collection. close inactive accounts with no liabilities. sending rent back / funds back to owner beyond the garbage collectors fee to at least breakeven. this fee could be increased over time to fit economics better but better to be nice initially.
• dynamic fee on user accounts that fit with dapp economics. (e.g. too many user accounts? start charging for account creation)
• an 'archive program'. where user accounts can be stored outside the main drift program to reduce strain on active system.