Retain input channel instances for lazily executed operators

[sekruse]

In some rather unusual situations, we might run into problems with the inputs of lazily executed operators:

1. Some channel instance, e.g., a file F, is produced.
2. A lazily executed operator O consumes that channel instance.
3. However, O is not actually executed within its stage for some reason.
4. In some subsequent stage, the execution of O is triggered but F is already deleted. The execution fails.

An obvious fix is to pinpoint F in the lazy execution lineage until the execution of O has actually taken place.