solver3: Use a propagation queue

Instead of directly propagating in a recursive fashion,
queue propagations in a queue and work on them in a loop
per the miniSAT paper.

We call Propagate() only at the end of the FromDepCache()
function and then in the Solve loop. Delaying the initial
propagation means that we get a stronger reasoning:

Assume you have x->a->b->c, y->c and you install x,y:
- Previously we traversed: x, y, x->a, a->b, b->c, (y->c)
- but now we traverse:     x, y, x->a, y->c, a->b, (b->c)
Notably c now has the implication y->c instead of x->a->b->c.

Inside the solver we need to call Propagate in a loop:
Propagating facts can fail and we then backtrack. If backtracking
is succesful, we have gained a new fact to propagate.

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