Track where microdescs are referenced to prevent free errs

On IRC, wanoskarnet notes that if we ever do microdesc_free() on a
microdesc that's in the nodelist, we're in trouble.  Also, we're in
trouble if we free one that's still in the microdesc_cache map.

This code adds a flag to microdesc_t to note where the microdesc is
referenced from, and checks those flags from microdesc_free().  I
don't believe we have any errors here now, but if we introduce some
later, let's log and recover from them rather than introducing
heisenbugs later on.

Addresses bug 3153.

changes/bug3153

@@ -0,0 +1,5 @@
+  o Minor features:
+    - Check for and recover from inconsistency in the microdescriptor
+      cache.  This will make it harder for us to accidentally free a
+      microdescriptor without removing it from the appropriate data
+      structures.  Fixes issue 3135; issue noted by wanoskarnet.