`buffer`, `interval`, `to_stream`, and `to_typed_stream` deadlock with a delay of `timespan::zero()`

[dominiklohmann]

The interval flow operator deadlocks when passing in caf::timespan::zero() for the max_delay argument.

We tracked it down to the loop in this code which never finishes when period_ <= timespan::zero():

actor-framework/libcaf_core/caf/flow/op/interval.cpp

Lines 72 to 77 in 06ed712

	auto now = parent_->steady_time();
	auto next = last_ + period_;
	while (next <= now)
	next += period_;
	last_ = next;
	schedule_next(next);

Besides the interval operator, this also affects the buffer operator and the to_stream and to_typed_stream methods on observables, which use interval internally.

I think the following changes should be made to fix the issues:

1. The interval and buffer should assert that max_delay >= timespan::zero() holds true.
2. The to_stream and to_typed_stream methods should support an unbuffered mode where they do not even attempt to create the interval operator.