FIP: Reduce Storage Limits

Farcaster proposes lowering per-unit storage limits for casts, reactions and links without changing storage unit prices, to slow the growth of hub data. Hub data is about 140GB today and grows 1GB/day, with a soft cap at 7 TB where high-performance NVMe disks tap out. Casts would drop from 5000 to 4000, and reactions and links from 2500 to 2000, an estimated 5% reduction in hub data size. About 1% of accounts, mostly programmatic ones, would lose casts and links, and about 5% would lose reactions; the new limits take effect 2024.08.08, one week after protocol version 2024.07.24.