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. The interface returns the amount of time to wait before applying
an oplog entry.

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.
Is this just for initialization purposes? If so, can it happen in `New`?