op-challenger: Provide option to limit how often games are processed

[ajsutton]

op-challenger makes a lot of calls to the L1 node because it's tracking all games from the last 28 days and for each game it winds up loading all the claims for those games. Currently it tries to update games basically as fast as it can - scheduling a new round of updates on each new block (though back pressure means it doesn't actually update that fast).

To reduce the number of calls it makes, it would be useful to provide an option to limit how often games are updated. Possibly the easiest way to do this is to only schedule processing games if the new L1 block is X seconds after the previous update was scheduled but can be flexible about the particular semantics.