Performance logging and counters:

* Tally and duration counters for Job Queue tasks and RPC calls
    optionally rendered by server_info and server_state, and
    optionally printed to a distinct log file.
    - Tally each Job Queue task as it is queued, starts, and
      finishes running. Track total duration queued and running.
    - Tally each RPC call as it starts and either finishes
      successfully or throws an exception. Track total running
      duration for each.
  * Track currently executing Job Queue tasks and RPC methods
    along with durations.
  * Json-formatted performance log file written by a dedicated
    thread, for above-described data.
  * New optional parameter, "counters", for server_info and
    server_state. If set, render Job Queue and RPC call counters
    as well as currently executing tasks.
  * New configuration section, "[perf]", to optionally control
    performance logging to a file.
  * Support optional sub-second periods when rendering human-readable
    time points.

src/ripple/core/JobQueue.h

@@ -31,6 +31,11 @@
 
 namespace ripple {
 
+namespace perf
+{
+    class PerfLog;
+}
+
 class Logs;
 struct Coro_create_t
 {
@@ -132,7 +137,8 @@ public:
     using JobFunction = std::function <void(Job&)>;
 
     JobQueue (beast::insight::Collector::ptr const& collector,
-        Stoppable& parent, beast::Journal journal, Logs& logs);
+        Stoppable& parent, beast::Journal journal, Logs& logs,
+        perf::PerfLog& perfLog);
     ~JobQueue ();
 
     /** Adds a job to the JobQueue.
@@ -226,18 +232,13 @@ private:
     Job::CancelCallback m_cancelCallback;
 
     // Statistics tracking
+    perf::PerfLog& perfLog_;
     beast::insight::Collector::ptr m_collector;
     beast::insight::Gauge job_count;
     beast::insight::Hook hook;
 
     std::condition_variable cv_;
 
-    static JobTypes const& getJobTypes()
-    {
-        static JobTypes types;
-        return types;
-    }
-
     void collect();
     JobTypeData& getJobTypeData (JobType type);
 
@@ -304,14 +305,6 @@ private:
     //  <none>
     void finishJob (JobType type);
 
-    template <class Rep, class Period>
-    void on_dequeue (JobType type,
-        std::chrono::duration <Rep, Period> const& value);
-
-    template <class Rep, class Period>
-    void on_execute (JobType type,
-        std::chrono::duration <Rep, Period> const& value);
-
     // Runs the next appropriate waiting Job.
     //
     // Pre-conditions:
@@ -322,7 +315,7 @@ private:
     //
     // Invariants:
     //  <none>
-    void processTask () override;
+    void processTask (int instance) override;
 
     // Returns the limit of running jobs for the given job type.
     // For jobs with no limit, we return the largest int. Hopefully that