resource: avoid segfault during teardown

Problem: unloading resource module with events posted to eventlog
in flight can resut in segfault.

Program terminated with signal SIGSEGV, Segmentation fault.

 #0  __strcmp_avx2 () at ../sysdeps/x86_64/multiarch/strcmp-avx2.S:102
 102     ../sysdeps/x86_64/multiarch/strcmp-avx2.S: No such file or directory.
 [Current thread is 1 (Thread 0x7fe74b7fe700 (LWP 3495430))]
 (gdb) bt
 #0  __strcmp_avx2 () at ../sysdeps/x86_64/multiarch/strcmp-avx2.S:102
 #1  0x00007fe764f40de0 in aux_item_find (key=<optimized out>,
     head=0x7fe73c006180) at aux.c:88
 #2  aux_get (head=<optimized out>, key=0x7fe764f5b000 "flux::log") at aux.c:119
 #3  0x00007fe764f1f0d4 in getctx (h=h@entry=0x7fe73c00c6d0) at flog.c:72
 #4  0x00007fe764f1f3a5 in flux_vlog (h=0x7fe73c00c6d0, level=7,
     fmt=0x7fe7606318fc "%s: %s event posted", ap=ap@entry=0x7fe74b7fd790)
     at flog.c:146
 #5  0x00007fe764f1f333 in flux_log (h=<optimized out>, lev=lev@entry=7,
    fmt=fmt@entry=0x7fe7606318fc "%s: %s event posted") at flog.c:195
 flux-framework#6  0x00007fe76061166a in reslog_cb (reslog=<optimized out>,
     name=0x7fe73c016380 "online", arg=0x7fe73c013000) at acquire.c:319
 flux-framework#7  0x00007fe760610deb in notify_callback (event=<optimized out>,
     reslog=0x7fe73c005b90) at reslog.c:47
 flux-framework#8  post_handler (reslog=reslog@entry=0x7fe73c005b90, f=0x7fe73c00a510)
     at reslog.c:91
 flux-framework#9  0x00007fe760611250 in reslog_destroy (reslog=0x7fe73c005b90)
     at reslog.c:182
 flux-framework#10 0x00007fe76060e6b8 in resource_ctx_destroy (ctx=ctx@entry=0x7fe73c016640)
     at resource.c:129
 flux-framework#11 0x00007fe76060ef18 in resource_ctx_destroy (ctx=0x7fe73c016640)
     at resource.c:331

It looks like the acquire subsystem got a callback for a rank coming online
after its context was freed.  Set the reslog callback to NULL before
destroying the acquire context.

Also, set the monitor callback to NULL before destroying the discover
context, as it appears this destructor has a similar safety issue.

src/modules/resource/acquire.c

@@ -379,6 +379,7 @@ void acquire_destroy (struct acquire *acquire)
     if (acquire) {
         int saved_errno = errno;
         flux_msg_handler_delvec (acquire->handlers);
+        reslog_set_callback (acquire->ctx->reslog, NULL, NULL);
         if (acquire->request) {
             if (flux_respond_error (acquire->ctx->h,
                                     acquire->request->msg,

src/modules/resource/discover.c

@@ -278,6 +278,7 @@ void discover_destroy (struct discover *discover)
 {
     if (discover) {
         int saved_errno = errno;
+        monitor_set_callback (discover->ctx->monitor, NULL, NULL);
         flux_subprocess_destroy (discover->p);
         flux_future_destroy (discover->f);
         flux_msg_handler_delvec (discover->handlers);