Support fixed rate oplog replaying (ops per second)

This change adds an interface for controlling the rate at which we apply
op logs. Theoretically this interface could control the rate at which
more generic things are done. Though it's similar to the leakybucket
abstraction we already have, the relative rate ideas we have didn't
cleanly map onto the leakybucket so I thought this new interface was
useful. I considered doing the interface slightly differently -
returning a time to wait until applying the operation instead of just a
boolean. I ended up going with this because it a slightly simpler
interface, but I could probably be convinced the other one is better.

This change also adds to implementations of the interface. The fixed and
relative rate controllers we want for oplogs. It also adds a new command
line parameter to control which rate controller we use in the replay. It
defaults to the fixed rate.
Good point. It could do something like take in a function to actually apply a (set of) ops in the respective `New` functions, and then it just needs to know when to invoke that function, which will be different for every `Controller`. That sounds more complicated, though - probably best to stick with the simple interface you have here, just changing it to return a `Duration`.