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+.
I believe sending on a channel is by-reference, but I could be wrong.

Personally, I would reallocate the slice unless we know that time spent allocating those slices is going to cause a big performance hit. This seems like a premature optimization with the questionable side-effect of preventing GC. That said, I think either is fine - my way isn't necessarily the best way ;)