Add inhibitor to queue before activation

[lightclient]

This PR implements a features first requested by @marioevz.

The concern is that before 7002 becomes active, it would be possible to join the queue without excess growing. This means the queue could be filled extremely cheaply.

To avoid this, we set the value of excess to an extremely high value so that no user could ever pay a fee to enter the queue. This avoids additional sloads during normal operations.

We chose 1181 as the value of excess because i) it a prime and ii) fake_exponential(1, 1181, 17) = 1481455745989958413761130954915 wei which is more than 1 trillion eth. That's more eth than will (hopefully) ever exist.

Each system call will check excess == 1181 and use 0 in it's place if so. This of course will only be true for the first call following activation of Prague.