Initialize the HS cache at startup

Failure to do this caused an assertion failure with #22246 . This
assertion failure can be triggered remotely, so we're tracking it as
medium-severity TROVE-2017-002.

changes/bug22246

@@ -0,0 +1,6 @@
+  o Major bugfixes (hidden service directory, security):
+    - Fix an assertion failure in the hidden service directory code, which
+      could be used by an attacker to remotely cause a Tor relay process to
+      exit. Relays running earlier versions of Tor 0.3.0.x should upgrade.
+      This security issue is tracked as tracked as
+      TROVE-2017-002. Fixes bug 22246; bugfix on 0.3.0.1-alpha.

src/or/main.c

@@ -2926,6 +2926,7 @@ tor_init(int argc, char *argv[])
   rep_hist_init();
   /* Initialize the service cache. */
   rend_cache_init();
+  hs_cache_init();
   addressmap_init(); /* Init the client dns cache. Do it always, since it's
                       * cheap. */