Allow subscription to peer status changes (RIPD-579)

Subscribe to "peer_status" stream (admin only) permits
reception of "peerStatusChange" notifications.

These can include the event the peer is reporting, the peer's
new status, the peer's currently accepted ledger hash and sequence,
the peer's network time, and the range of ledgers the peer has
available for remote querying.

src/ripple/net/impl/InfoSub.cpp

@@ -58,6 +58,7 @@ InfoSub::~InfoSub ()
     m_source.unsubLedger (mSeq);
     m_source.unsubServer (mSeq);
     m_source.unsubValidations (mSeq);
+    m_source.unsubPeerStatus (mSeq);
 
     // Use the internal unsubscribe so that it won't call
     // back to us and modify its own parameter