Batch oplog requests

 Before this change when replaying the oplog we maxed out at ~150
requests per second. We think the bottleneck was the round trip time
between the database and the oplog-replay script. To address that issue
we're supporting batching oplog requests.
    
Conceptually we still have one goroutine responsible for putting the ops
in the "channel" to be processed, but now the goroutine that actual calls
mongo to apply to operation applies everything currently in the queue at
once.

It should be noted that oplog requests will only be batched if the replay is
bottlenecked on round trip time to the db. Otherwise the requests will be
sent as soon as they should.