Load PeerFinder database in Stoppable::onPrepare:

OverlayImpl::onStart calls into PeerFinder before PeerFinder::Manager::onStart,
causing tests to sometimes fail and the application to intermittently not start.
The order of calls to Stoppable::onStart is implementation defined and not
predictable.

This changes PeerFinder to load the database in Stoppable::onPrepare, before
threads are launched. In general, creation and initialization of resources that
are shared between classes should happen in onPrepare rather than onStart,
to solve this problem.

src/ripple/peerfinder/impl/Manager.cpp

@@ -201,14 +201,9 @@ public:
     //
     //--------------------------------------------------------------------------
 
-    void onPrepare ()
-    {
-    }
-
     void
-    onStart()
+    onPrepare ()
     {
-        m_journal.debug << "Initializing";
         beast::Error error (m_store.open (m_databaseFile));
         if (error)
             m_journal.fatal <<
@@ -217,6 +212,11 @@ public:
             m_logic.load ();
     }
 
+    void
+    onStart()
+    {
+    }
+
     void onStop ()
     {
         m_journal.debug << "Stopping";