Don't delay descriptor fetches when missing info needed for circuits

When we have fewer than 15 descriptors to fetch, we will delay the
fetch for a little while.  That's fine, if we can go ahead and build
circuits... but if not, it's a poor choice indeed.

Fixes bug 23985; bugfix on 0.1.1.11-alpha.

In 0.3.0.3-alpha, when we made primary guard descriptors necessary
for circuit building, this situation got worse.

src/or/routerlist.c

@@ -5035,6 +5035,11 @@ launch_descriptor_downloads(int purpose,
       log_debug(LD_DIR,
                 "There are enough downloadable %ss to launch requests.",
                 descname);
+    } else if (! router_have_minimum_dir_info()) {
+      log_debug(LD_DIR,
+                "We are only missing %d %ss, but we'll fetch anyway, since "
+                "we don't yet have enough directory info.",
+                n_downloadable, descname);
     } else {
 
       /* should delay */