Revisit iteration styles

[fpacifici]

Reflect on the different iteration styles into_iter, iter, iter_mut, vs &, &mut.
for example here:

rust_arroyo/src/backends/local/broker.rs

Line 75 in 1ce4b64

for topic in topics.iter() {

you take the argument as an allocated Vec, but you only iterate over references of its members via .iter, afterwards freeing that allocation. Better to either take a &[T] as above, or consume the vector via into_iter (or just … in topics as that will also consume the vec)