Refactor replay

Refactor the replay package.  This change breaks the replay into well separated pipeline stages, namely:

[parse BSON] ---op---> [controlRate] ---op---> [batchOps] ---[]op---> [applyOps]

Each stage controls the lifetime of their own output channels, closing them when appropriate.  Each stage also properly exits early in the case of failure.  The design of the new package is influenced heavily by [this blog post on pipelines](http://blog.golang.org/pipelines)
This update also makes oplog-replay compatible with go 1.3+.
If you're looking for a different way to do batching, I really like this way: https://gist.github.com/azylman/cc3aded732000c085780

I'm not sure if that could be adapted to work with the `RateController` here, though.