PERF: avoid spinning a thread each time we close a connection

This is a temporary workaround for the issue in https://github.com/rails/rails/pull/36949

Discussing a proper fix in Rails with the Rails team.

Prior to this fix we were spinning up a thread every time we closed a connection
to the db.

script/thread_detective.rb

@@ -0,0 +1,56 @@
+# frozen_string_literal: true
+
+class Thread
+  attr_accessor :origin
+end
+
+class ThreadDetective
+  def self.test_thread
+    Thread.new { sleep 1 }
+  end
+  def self.start(max_threads)
+    @thread ||= Thread.new do
+      self.new.monitor(max_threads)
+    end
+
+    @trace = TracePoint.new(:thread_begin) do |tp|
+      Thread.current.origin = Thread.current.inspect
+    end
+    @trace.enable
+  end
+
+  def self.stop
+    @thread&.kill
+    @thread = nil
+    @trace&.disable
+    @trace.stop
+  end
+
+  def monitor(max_threads)
+    STDERR.puts "Monitoring threads in #{Process.pid}"
+
+    while true
+      threads = Thread.list
+
+      if threads.length > max_threads
+        str = +("-" * 60)
+        str << "#{threads.length} found in Process #{Process.pid}!\n"
+
+        threads.each do |thread|
+          str << "\n"
+          if thread.origin
+            str << thread.origin
+          else
+            str << thread.inspect
+          end
+          str << "\n"
+        end
+        str << ("-" * 60)
+
+        STDERR.puts str
+      end
+      sleep 1
+    end
+  end
+
+end