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+.
There's a lot of repeated code in this case and the default case. Is there a better way to handle this? Two things I'm wondering about: checking for the `done` channel first, before trying to read from `ops`, so you didn't need to check in both cases here and making the `applyOps` pipeline stage support empty batches (i.e. do nothing) so you didn't need to check that either.