Fix build references to deleted ServerHandlerImp: (#4592)

* Commits 0b812cd (#4427) and 11e914f (#4516) conflict. The first added
  references to `ServerHandlerImp` in files outside of that class's
  organizational unit (which is technically incorrect). The second
  removed `ServerHandlerImp`, but was not up to date with develop. This
  results in the build failing.
* Fixes the build by changing references to `ServerHandlerImp` to
  the more correct `ServerHandler`.

src/ripple/app/main/Application.cpp

@@ -602,7 +602,7 @@ public:
         return *m_networkOPs;
     }
 
-    virtual ServerHandlerImp&
+    virtual ServerHandler&
     getServerHandler() override
     {
         assert(serverHandler_);

src/ripple/app/main/Application.h

@@ -89,7 +89,7 @@ class Overlay;
 class PathRequests;
 class PendingSaves;
 class PublicKey;
-class ServerHandlerImp;
+class ServerHandler;
 class SecretKey;
 class STLedgerEntry;
 class TimeKeeper;
@@ -232,7 +232,7 @@ public:
     getOPs() = 0;
     virtual OrderBookDB&
     getOrderBookDB() = 0;
-    virtual ServerHandlerImp&
+    virtual ServerHandler&
     getServerHandler() = 0;
     virtual TransactionMaster&
     getMasterTransaction() = 0;

src/ripple/app/misc/NetworkOPs.cpp

@@ -64,8 +64,8 @@
 #include <ripple/resource/ResourceManager.h>
 #include <ripple/rpc/BookChanges.h>
 #include <ripple/rpc/DeliveredAmount.h>
+#include <ripple/rpc/ServerHandler.h>
 #include <ripple/rpc/impl/RPCHelpers.h>
-#include <ripple/rpc/impl/ServerHandlerImp.h>
 #include <boost/asio/ip/host_name.hpp>
 #include <boost/asio/steady_timer.hpp>