This changes behaviour somewhat in that:
- We now consistently release the pool on shutdown, even on non-Unix
platforms, and including for CmdDumpConfig.
- We release the pool *after* interrupting `App.run`, which will
rather cause more than fewer connection to be closed properly.
(Previously any in-use connections would not have been caught by
`releasePool`, though *maybe* the `UserInterrupt` handling in
the web handler ends up closing the connections properly already
anyway).
(The main aim of the change is to make it clearer when and why the
pool is released.)
