Reduce PING frequency and simplify logic:

The existing code issues a PING to each peer every 8 seconds. While
frequent PINGs allow us to estimate a peer's latency with a high
degree of accuracy, this "inter-server polka dance" is inefficient
and not useful. This commit, if merged, reduces the PING frequency
to once every 60 seconds.

Additionally, this commit simplifies the PING handling logic and
merges the code used to check and disconnect peers which fail to
track the network directly into the timer callback.

src/ripple/overlay/impl/OverlayImpl.cpp

@@ -100,9 +100,6 @@ OverlayImpl::Timer::on_timer(error_code ec)
     overlay_.sendEndpoints();
     overlay_.autoConnect();
 
-    if ((++overlay_.timer_count_ % Tuning::checkSeconds) == 0)
-        overlay_.check();
-
     if ((overlay_.timer_count_ % Tuning::checkIdlePeers) == 0)
         overlay_.deleteIdlePeers();
 
@@ -1187,12 +1184,6 @@ OverlayImpl::checkTracking(std::uint32_t index)
         [index](std::shared_ptr<PeerImp>&& sp) { sp->checkTracking(index); });
 }
 
-void
-OverlayImpl::check()
-{
-    for_each([](std::shared_ptr<PeerImp>&& sp) { sp->check(); });
-}
-
 std::shared_ptr<Peer>
 OverlayImpl::findPeerByShortID(Peer::id_t const& id) const
 {