bug(partition): Subscribes to the source twice, when it should probably multicast

[benlesh]

Currently partition will subscribe to whatever observable you get twice. Given that it is supposed to "partition", which sounds like splitting one thing into several parts, I'd think it should multicast the source.

ALSO: The partition operator just needs to be removed in favor of groupBy, as it doesn't make any sense as an operator with pipe.

[cartant]

Related: #3807

And there is a splitBy in rxjs-etc that I mentioned in this comment.

[benlesh]

This is still an issue with the existing static partition, and we should address it sometime. It's debatable as to whether it's a breaking change.

[Ayudh]

@benlesh can I work on this?