cmd: extend default timeout when needed in flux overlay errors

Problem: On systems with ~10K nodes, `flux overlay errors` sometimes
reports "Connection timed out" for some ranks for which RPCs are
issued on the first iteration. The problem seems to be that the
timeout starts immediately when `flux_future_then(3)` is called, but
for large systems with a flat TBON the program may not re-enter the
reactor for >0.5s due to the size of the initial payload.

While one solution would be to delay sending _any_ RPCs until the
first time the check watcher is called, this unnecessarily extends
the runtime of the program by at least the initial payload processing
time. Instead, scale the timeout for large systems (>2K nodes) by
the size of system, such that 10K node systems get a roughly 2.5s
timeout, which seems to be a safe value.

Note that a long timeout is not as much of a problem as in previous
versions of the program where overlay.health RPCs were sent serially,
since the longer timeout can now happen in parallel.

src/cmd/builtin/overlay.c

@@ -1089,8 +1089,18 @@ static int subcmd_errors (optparse_t *p, int ac, char *av[])
 {
     flux_t *h = builtin_get_flux_handle (p);
     struct overlay_errors *ctx = NULL;
+    uint32_t size;
     double timeout;
 
+    /* On large systems with a flat tbon, the default timeout of 0.5s
+     * may not be long enough because the program may spend a long time
+     * simply processing the initial JSON response payload for rank 0, and
+     * meanwhile the then timeout is ticking. Therefore, if the current
+     * instance is large, scale the timeout by the instance size:
+     */
+    if (flux_get_size (h, &size) == 0 && size > 2048)
+        default_timeout *= (size / 2000.); /* ~2.5s timeout on 10K nodes */
+
     timeout = optparse_get_duration (p, "timeout", default_timeout);
     if (timeout == 0)
         timeout = -1.0; // disabled