Cleanups in beast::Journal:

The Journal API is affected.  There are two uses for the
Journal::Severity enum:

 o It is used to declare a threshold which log messages must meet
   in order to be logged.

 o It declares the current logging level which will be compared
   to the threshold.

Those uses that affect the threshold are now named threshold()
rather than severity() to make the uses easier to distinguish.

Additionally, Journal no longer carries a Severity variable.
All handling of the threshold() is now delegated to the
Journal::Sink.

Sinks are no longer constructed with a default threshold of
kWarning; their threshold must be passed in on construction.

src/ripple/basics/impl/Log.cpp

@@ -28,16 +28,19 @@
 namespace ripple {
 
 Logs::Sink::Sink (std::string const& partition,
-    beast::Journal::Severity severity, Logs& logs)
-    : logs_(logs)
+    beast::Journal::Severity thresh, Logs& logs)
+    : beast::Journal::Sink (thresh, false)
+    , logs_(logs)
     , partition_(partition)
 {
-    beast::Journal::Sink::severity (severity);
 }
 
 void
 Logs::Sink::write (beast::Journal::Severity level, std::string const& text)
 {
+    if (level < threshold())
+        return;
+
     logs_.write (level, partition_, text, console());
 }
 
@@ -109,7 +112,7 @@ void Logs::File::writeln (char const* text)
 //------------------------------------------------------------------------------
 
 Logs::Logs()
-    : level_ (beast::Journal::kWarning) // default severity
+    : thresh_ (beast::Journal::kWarning) // default severity
 {
 }
 
@@ -124,7 +127,7 @@ Logs::get (std::string const& name)
 {
     std::lock_guard <std::mutex> lock (mutex_);
     auto const result =
-        sinks_.emplace(name, makeSink(name, level_));
+        sinks_.emplace(name, makeSink(name, thresh_));
     return *result.first->second;
 }
 
@@ -141,18 +144,18 @@ Logs::journal (std::string const& name)
 }
 
 beast::Journal::Severity
-Logs::severity() const
+Logs::threshold() const
 {
-    return level_;
+    return thresh_;
 }
 
 void
-Logs::severity (beast::Journal::Severity level)
+Logs::threshold (beast::Journal::Severity thresh)
 {
     std::lock_guard <std::mutex> lock (mutex_);
-    level_ = level;
+    thresh_ = thresh;
     for (auto& sink : sinks_)
-        sink.second->severity (level);
+        sink.second->threshold (thresh);
 }
 
 std::vector<std::pair<std::string, std::string>>
@@ -163,7 +166,7 @@ Logs::partition_severities() const
     list.reserve (sinks_.size());
     for (auto const& e : sinks_)
         list.push_back(std::make_pair(e.first,
-            toString(fromSeverity(e.second->severity()))));
+            toString(fromSeverity(e.second->threshold()))));
     return list;
 }
 
@@ -194,10 +197,10 @@ Logs::rotate()
 
 std::unique_ptr<beast::Journal::Sink>
 Logs::makeSink(std::string const& name,
-    beast::Journal::Severity startingLevel)
+    beast::Journal::Severity threshold)
 {
     return std::make_unique<Sink>(
-        name, startingLevel, *this);
+        name, threshold, *this);
 }
 
 LogSeverity