Simplify the Job Queue:

This is a refactor aimed at cleaning up and simplifying the existing
job queue.

As of now, all jobs are cancelled at the same time and in the same
way, so this commit removes the per-job cancellation token. If the
need for such support is demonstrated, support can be re-added.

* Revise documentation for ClosureCounter and Workers.
* Simplify code, removing unnecessary function arguments and
  deduplicating expressions
* Restructure job handlers to no longer need to pass a job's
  handle to the job.

src/ripple/core/impl/Job.cpp

@@ -36,10 +36,8 @@ Job::Job(
     std::string const& name,
     std::uint64_t index,
     LoadMonitor& lm,
-    std::function<void(Job&)> const& job,
-    CancelCallback cancelCallback)
-    : m_cancelCallback(cancelCallback)
-    , mType(type)
+    std::function<void()> const& job)
+    : mType(type)
     , mJobIndex(index)
     , mJob(job)
     , mName(name)
@@ -54,27 +52,12 @@ Job::getType() const
     return mType;
 }
 
-Job::CancelCallback
-Job::getCancelCallback() const
-{
-    assert(m_cancelCallback);
-    return m_cancelCallback;
-}
-
 Job::clock_type::time_point const&
 Job::queue_time() const
 {
     return m_queue_time;
 }
 
-bool
-Job::shouldCancel() const
-{
-    if (m_cancelCallback)
-        return m_cancelCallback();
-    return false;
-}
-
 void
 Job::doJob()
 {
@@ -82,19 +65,13 @@ Job::doJob()
     m_loadEvent->start();
     m_loadEvent->setName(mName);
 
-    mJob(*this);
+    mJob();
 
     // Destroy the lambda, otherwise we won't include
     // its duration in the time measurement
     mJob = nullptr;
 }
 
-void
-Job::rename(std::string const& newName)
-{
-    mName = newName;
-}
-
 bool
 Job::operator>(const Job& j) const
 {