[net] Add RunInactivityChecks()

Moves the logic to prevent running inactivity checks until
the peer has been connected for -peertimeout time into its
own function. This will be reused by net_processing later.

src/net.h

@@ -1022,6 +1022,9 @@ public:
 
     void SetAsmap(std::vector<bool> asmap) { addrman.m_asmap = std::move(asmap); }
 
+    /** Return true if the peer has been connected for long enough to do inactivity checks. */
+    bool RunInactivityChecks(const CNode& node) const;
+
 private:
     struct ListenSocket {
     public: