Support for sharded clusters

[benweissmann]

We can probably support sharded mongo clusters by just ignoring oplog entries with fromMigrate (https://www.mongodb.com/blog/post/tailing-mongodb-oplog-sharded-clusters) and running a separate oplogtoredis for each shard, but this needs testing.

This is likely a low-priority task, unless there's someone who'd like to use oplogtoredis on a sharded cluster and is willing to do the testing component of this task.

[radekmie]

Would having that result in Meteor's compatibility with sharded databases? That would be a huge step -- maybe "huge enough" for some more traction towards this great piece of software.