What is the additional gas cost of the OpenZeppelin ReentrancyGuard?

[nventuro]

@frangio

@nventuro and @DericEcourcy recently revamped ReentrancyGuard to be cheaper when the upcoming net gas metering lands.

They may have some insights into what the gas costs will be with this change.

The overhead should be 4 SLOAD, 5 if the compiler is not smart. SLOADs are being repriced from 200 to 800 however, so I'd expect ~3.5k gas.