Use saturating_add in shared_object_congestion_tracker.rs

[muXxer]

SUI fixed some overflows in the shared object congestion tracker.
We should fix those as well, and go through the rest of the code base if there are similar bugs.

MystenLabs/sui@837bf41
MystenLabs/sui@a139ef2

I'm wondering about the change with the <=, I think they refer to that with 0-cost tx, but what exactly is a 0-cost tx?

[cuileri]

I'm wondering about the change with the <=, I think they refer to that with 0-cost tx, but what exactly is a 0-cost tx?

Since the active cost mode is the GasBudget mode, one could specify a high budget to cause overflow. Since they used saturating_add to address this issue, now it's possible that a transaction will not increase an already saturated number. So the problem is not 0-cost transactions but rather continuing to evaluate transactions after saturation occurred.