Reduce the number of indexes in o_* collections

[gkubisa]

ShareDB stores all operations in collections prefixed with o_. Currently those collections have 3 indexes: _id, documentId + version, src + seq. I think we could have only one index, on _id, which MongoDB always creates automatically.

To keep the most common queries fast, we'd need to update IDs of all operations to this format: <documentId>:<version>:<randomValue>. This way we would not need the separate compound index documentId + version.

The other compound index src + seq is used so rarely that I don't think it's worth having. The code currently using this index could be modified to take advantage of the _id index to reduce the number of docs that would need to be scanned.

[alecgibson]

We've had a discussion about this and we agree about src + seq, but think that we can't combine documentId + version without left-padding version (because we need version range lookups). Left-padding feels a bit hacky and fragile, and documentId + version feels like a reasonable index to have, so we'll leave that one as is