broker: flush content cache on request

Problem: The content.flush target does not actually flush the
cache.  It only waits for current flushes to complete.  This
goes against documentation and can lead to issues when a cache
flush has not yet initiated.

Solution: Call cache_flush() from within content_flush_request().

Fixes flux-framework#4378

src/broker/content-cache.c

@@ -774,6 +774,8 @@ static void content_flush_request (flux_t *h, flux_msg_handler_t *mh,
     struct content_cache *cache = arg;
 
     if (cache->acct_dirty > 0) {
+        if (cache_flush (cache) < 0)
+            goto error;
         if (msgstack_push (&cache->flush_requests, msg) < 0)
             goto error;
         return;