Close minor startup race between main and scheduler threads

Don't schedule class PeerManagerImpl's background tasks from its
constructor, but instead do that from a separate method,
StartScheduledTasks(), that can be called later at the end of startup,
after other things, such as the active chain, are initialzed.

src/net_processing.h

@@ -37,10 +37,13 @@ class PeerManager : public CValidationInterface, public NetEventsInterface
 {
 public:
     static std::unique_ptr<PeerManager> make(const CChainParams& chainparams, CConnman& connman, CAddrMan& addrman,
-                                             BanMan* banman, CScheduler& scheduler, ChainstateManager& chainman,
+                                             BanMan* banman, ChainstateManager& chainman,
                                              CTxMemPool& pool, bool ignore_incoming_txs);
     virtual ~PeerManager() { }
 
+    /** Begin running background tasks, should only be called once */
+    virtual void StartScheduledTasks(CScheduler& scheduler) = 0;
+
     /** Get statistics from node state */
     virtual bool GetNodeStateStats(NodeId nodeid, CNodeStateStats& stats) const = 0;