.distinct operator needs to accept external store

[bittrance]

In its current form, the .distinct operator uses an internal Hash to keep track of seen keys. This may be a good default, but does not scale for extensive or long-running streams. RxJS solves this by allowing to provide a "flush" observable, but purging the entire memory is not alwsys acceptable. Rather, .distinct should accept a hash-like object as input so that the user can implement a suitable pruning strategy or even have a memory backed by disk. This would also allow multiple streams to coordinate distinctiveness without merging streams.