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.
Change the interface

We want to use the WaitTime() (time.Duration) interface because it's a
bit cleaner.
Merge pull request #13 from Clever/ops-per-second

Support fixed rate oplog replaying (ops per second)