Fix cyclic graph state explosion

[CapCap]

When states cycle A->B->A->B, there are a truly exponential number of states that get created. Two such cycles are enough to cause 10s + loading times for pages, and realistically will easily break altogether.

Big thank you to @JohnieBraaf for the fixes here.

This should fix https://github.com/javrasya/django-river/issues/188 and https://github.com/javrasya/django-river/issues/120

The problem is discussed in detail in the above issues.

[JohnieBraaf]

Thank you for creating this PR @CapCap

[pupubird]

Any time soon to have this fix on the codebase? Thanks

[CapCap]

Unlikely, author believes this is a feature not a bug, despite performance quickly exponentially declining when used

[pupubird]

Ah I see! No worries