Add reset-executor! and shutdown-executor! to re-frame.interop

[lowecg]

For #809

Executor Service Lifecycle Management
Added two functions to manage the executor service lifecycle in re-frame/interop:

reset-executor! - Creates a fresh single-threaded executor when needed
shutdown-executor! - Cleanly terminates the executor with configurable timeout

Why?
These functions enable proper cleanup in short-lived processes like AWS Lambda, CLI runs and tests. Without explicit shutdown, the non-daemon executor threads prevent the JVM from exiting cleanly.

Adding the following to a test ensures clean shutdown:

(ns my-ns
  (:require
    [clojure.test :refer :all]
    [re-frame.interop :as interop]
    ...))

(use-fixtures :once
              (fn [f]
                (f)  ; runs all tests
                (interop/shutdown-executor! 100)))