Fix Replicator's Reset Checkpoint not working as expected

Now the C4Replicator can be restarted so setting @kC4ReplicatorResetCheckpoint in the 

C4ReplicatorParameters when creating the C4Replicator doesn't make sense anymore.  

From Jens Alfke:

The kC4ReplicatorResetCheckpoint option no longer makes much sense, since the C4Replicator can be started multiple times. How about instead we add a bool resetCheckpoint parameter to c4repl_start()?