Replace DeadlineTimer with asio::steadyTimer (RIPD-1356):

The two active users of DeadlineTimer, NetworkOPs and Application,
now use asio::steady_timers rather than DeadlineTimer.
DeadlineTimer is removed since it is no longer used.

To assure that all in-flight closures on timers are done before
Stoppables call stopped(), the JobCounter is made more generic.
It's now a ClosureCounter.  The ClosureCounter is currently used
to count closures in flight for the JobQueue, NetworkOPs, and the
Application.

src/test/server/ServerStatus_test.cpp

@@ -25,7 +25,6 @@
 #include <test/jtx/envconfig.h>
 #include <test/jtx/WSClient.h>
 #include <test/jtx/JSONRPCClient.h>
-#include <ripple/core/DeadlineTimer.h>
 #include <ripple/app/misc/NetworkOPs.h>
 #include <ripple/app/ledger/LedgerMaster.h>
 #include <beast/http.hpp>