Incremental backups

[ioggstream]

It would be nice to support incremental backups, eg.

• First release: v1 only for replicasets.

--incremental-base $PATH_TO_UNTARRED_FULL_BACKUP

• Final release: v2 with shards.

--incremental-base $PATH_TO_INCREMENTAL_BACKUP

The untarred path should contain some metadata (eg. shards, previous backups, last oplog for each shard...)

If you are interested in the feature and provide some guidance I'm willing to contribute.

[dbmurphy]

@ioggstream You might check out our recent addition on Zbackup, as it will allow for some of the deduplication to save space, the next logical step would be to add a new tool that continuously is tailing all of the oplogs however I think this should be in go vs. python.