dirmgr: Call shrink_to_fit on missing-microdesc hashtable

This hashtable starts out pretty large, but it can spend most of our
runtime (when we aren't downloading) being small.  To avoid doing
too much work, I've made it so we only call shrink_to_fit twice per
consensus: once when we're no longer pending, and once when we're
complete.

Closes #388.

crates/tor-dirmgr/src/state.rs

@@ -764,6 +764,13 @@ impl<DM: WriteNetDir> DirState for GetMicrodescsState<DM> {
         if self.register_microdescs(microdescs) {
             // Just stopped being pending.
             self.mark_consensus_usable(storage)?;
+            // We can save a lot of ram this way, though we don't want to do it
+            // so often.  As a compromise we call `shrink_to_fit at most twice
+            // per consensus: once when we're ready to use, and once when we are
+            // complete.
+            self.missing.shrink_to_fit();
+        } else if self.missing.is_empty() {
+            self.missing.shrink_to_fit();
         }
 
         Ok(changed)