Prevent low-likelihood crash on shutdown (RIPD-1392):

The DatabaseImp has threads that asynchronously call JobQueue to
perform database reads.  Formerly these threads had the same
lifespan as Database, which was until the end-of-life of
ApplicationImp.  During shutdown these threads could call JobQueue
after JobQueue had already stopped.  Or, even worse, occasionally
call JobQueue after JobQueue's destructor had run.

To avoid these shutdown conditions, Database is made a Stoppable,
with JobQueue as its parent.  When Database stops, it shuts down
its asynchronous read threads.  This prevents Database from
accessing JobQueue after JobQueue has stopped, but allows
Database to perform stores for the remainder of shutdown.

During development it was noted that the Database::close()
method was never called.  So that method is removed from Database
and all derived classes.

Stoppable is also adjusted so it can be constructed using either
a char const* or a std::string.

For those files touched for other reasons, unneeded #includes
are removed.

src/ripple/core/JobQueue.h

@@ -22,19 +22,12 @@
 
 #include <ripple/basics/LocalValue.h>
 #include <ripple/basics/win32_workaround.h>
-#include <ripple/core/Job.h>
 #include <ripple/core/JobTypes.h>
 #include <ripple/core/JobTypeData.h>
+#include <ripple/core/Stoppable.h>
 #include <ripple/core/impl/Workers.h>
 #include <ripple/json/json_value.h>
-#include <ripple/beast/insight/Collector.h>
-#include <ripple/core/Stoppable.h>
 #include <boost/coroutine/all.hpp>
-#include <boost/function.hpp>
-#include <condition_variable>
-#include <mutex>
-#include <set>
-#include <thread>
 
 namespace ripple {