Optimize `resolve-missing`

[vemv]

1399da2 worked nicely for namespace-aliases, I definitely perceived an improvement in cljr-slash perf in my day job codebase.

Now resolve-missing is another of the big features offered by refactor-nrepl and deserves a similar treatment.

refactor-nrepl.ns.slam.hound.regrow is the relevant ns.

Notes

• (mapcat cp/classpath-seq (cp/classpath)) fails on non-existing jars orchard#125 is a blocker
• there's a double delay
  • this means some missing test coverage
• the warm-ast-cache op could @ all delays, for improving performance