Reduce number of dfu_impl_t::select calls

[milroy]

Depending on the match_op_t, dfu_impl_t::select can be called many times in dfu_traverser_t::schedule: https://github.com/flux-framework/flux-sched/blob/a9c1bd5d4d9058d148bf5b5cc6a737c28dc1ce4a/resource/traversers/dfu.cpp#L62

We should optimize and simplify the function logic to reduce the number of dfu_impl_t::select calls to reduce the number of traversals per schedule call.