Limit funding and interest accrual during downtimes (#529)

Previously, if the funding or interest updating instruction wasn't
called for a long time (like for a solana downtime or the security
council halting the program), the next update would apply funding or
interest for the whole time interval since the last update.

This could lead to a bad downtime situation becoming worse. Instead,
limit the maximum funding and interest time interval to one hour.

programs/mango-v4/src/instructions/token_update_index_and_rate.rs

@@ -71,7 +71,12 @@ pub fn token_update_index_and_rate(ctx: Context<TokenUpdateIndexAndRate>) -> Res
     {
         let mut some_bank = ctx.remaining_accounts[0].load_mut::<Bank>()?;
 
-        let diff_ts = I80F48::from_num(now_ts - some_bank.index_last_updated);
+        // Limit the maximal time interval that interest is applied for. This means we won't use
+        // a fixed interest rate for a very long time period in exceptional circumstances, like
+        // when there is a solana downtime or the security council disables this instruction.
+        let max_interest_timestep = 3600; // hour
+        let diff_ts =
+            I80F48::from_num((now_ts - some_bank.index_last_updated).min(max_interest_timestep));
 
         let (deposit_index, borrow_index, borrow_fees, borrow_rate, deposit_rate) =
             some_bank.compute_index(indexed_total_deposits, indexed_total_borrows, diff_ts)?;

programs/mango-v4/src/state/perp_market.rs

@@ -288,7 +288,14 @@ impl PerpMarket {
             (None, None) => I80F48::ZERO,
         };
 
-        let diff_ts = I80F48::from_num(now_ts - self.funding_last_updated as u64);
+        // Limit the maximal time interval that funding is applied for. This means we won't use
+        // the funding rate computed from a single orderbook snapshot for a very long time period
+        // in exceptional circumstances, like a solana downtime or the security council disabling
+        // funding updates.
+        let max_funding_timestep = 3600; // one hour
+        let diff_ts =
+            I80F48::from_num((now_ts - self.funding_last_updated as u64).min(max_funding_timestep));
+
         let time_factor = diff_ts / DAY_I80F48;
         let base_lot_size = I80F48::from_num(self.base_lot_size);