The specification of `split` should require that the input sender is only connected once.

[lewissbaker]

@griwes mentioned that the current wording of execution::split allows customisations to execute the input operation multiple times. This seems counter to the point of this operation and the default implementation only connects the input sender once when it's called.

I think we should restrict customisations to only connect() the input operation once.
i.e. make sure it's semantically equivalent to the default implementation