Change who calls microdesc_cache_rebuild().

Previously we ensured that it would get called periodically by doing
it from inside the code that added microdescriptors.  That won't work
though: it would interfere with our code that tried to read microdescs
from disk initially.  Instead, we should consider rebuilding the cache
periodically, and on startup.

changes/bug2230_part4

@@ -0,0 +1,6 @@
+  o Minor bugfixes:
+    - Authorities now clean their microdesc cache periodically and when
+      reading from disk initially, not only when adding new descriptors.
+      This prevents a bug where we could lose microdescriptors.  Bugfix
+      on 0.2.2.6-alpha.
+