websocket dialer hang on shutdown

[gdamore]

We're seeing a blocking reap --

    frame #2: 0x000000010004b204 ws_test`nni_pthread_cond_wait(c=0x0000000135706b70, m=0x0000000135706b30) at posix_thread.c:121:12
    frame #3: 0x000000010004b1d8 ws_test`nni_plat_cv_wait(cv=0x0000000135706b70) at posix_thread.c:229:2
    frame #4: 0x00000001000405b0 ws_test`nni_cv_wait(cv=0x0000000135706b70) at thread.c:51:2
    frame #5: 0x0000000100065bbc ws_test`ws_dialer_free(arg=0x0000000135706af0) at websocket.c:2269:3
    frame #6: 0x000000010003d4d4 ws_test`nng_stream_dialer_free(d=0x0000000135706af0) at stream.c:152:3
    frame #7: 0x0000000100056da8 ws_test`wstran_dialer_fini(arg=0x0000000135706a70) at websocket.c:378:2
    frame #8: 0x000000010002fe80 ws_test`nni_dialer_destroy(d=0x000000013600a000) at dialer.c:42:3
    frame #9: 0x000000010003bf4c ws_test`dialer_reap(arg=0x000000013600a000) at socket.c:1600:2
    frame #10: 0x0000000100038564 ws_test`reap_worker(unused=0x0000000000000000) at reap.c:58:5
    frame #11: 0x0000000100040860 ws_test`nni_thr_wrap(arg=0x0000000100075e18) at thread.c:94:3
    frame #12: 0x000000010004b4d8 ws_test`nni_plat_thr_main(arg=0x0000000100075e18) at posix_thread.c:266:2
    frame #13: 0x00000001864fbfa8 libsystem_pthread.dylib`_pthread_start + 148
  thread #24, name = 'nng:aio:expire'
    frame #0: 0x00000001864bf710 libsystem_kernel.dylib`__psynch_cvwait + 8
    frame #1: 0x00000001864fc574 libsystem_pthread.dylib`_pthread_cond_wait + 1232

This is waiting for the d->wspend (pending connections, i.e. those still negotiating) to drop to empty.

Probably we should just forcibly tear them down, when the dialer is shutting down.

A deferred clean up (but not blocking inside reap!) might be preferrable as well.