We log when we stop having enough dir info, but we never log when we resume having enough

When something goes wrong with our ability to use the network, we get a notice-level log like this one:

Oct 23 05:21:03.818 [notice] Our directory information is no longer up-to-date enough to build circuits: We're missing descriptors for 1/2 of our primary entry guards (total microdescriptors: 6432/6432). That's ok. We will try to fetch missing descriptors soon.

But when things recover, that message is only at info-level:

Oct 23 05:21:04.822 [info] We now have enough directory information to build circuits.

The original plan is described in the comments in update_router_have_minimum_dir_info():

    /* a) make us log when we next complete a circuit, so we know when Tor
     * is back up and usable, and b) disable some activities that Tor
     * should only do while circuits are working, like reachability tests
     * and fetching bridge descriptors only over circuits. */
    note_that_we_maybe_cant_complete_circuits();

but then we lost that notice-level log in commit eee62e13 (in Tor 0.3.5.1-alpha):

-    log_notice(LD_GENERAL,
-               "Tor has successfully opened a circuit. "
-               "Looks like client functionality is working.");
[...]
+    log_info(LD_GENERAL,
+             "Tor has successfully opened a circuit. "
+             "Looks like client functionality is working.");

as part of the shift in #14950 (closed) to reduce log clutter.

We should make sure there is a corresponding "we're back" message for each "we can't work yet" message that we write.