Fix global IO pool on JRuby

Use CachedThreadPool instead of configured ThreadPoolExecutor
to make sure that arriving jobs are passed to newly created threads
instead put into queue when there are no idle threads. Prevents deadlocking
when jobs depend on each other.

Author: pitr-ch

lib/concurrent/configuration.rb

@@ -175,13 +175,8 @@ def self.new_fast_executor(opts = {})
   end
 
   def self.new_io_executor(opts = {})
-    ThreadPoolExecutor.new(
-        min_threads:     [2, Concurrent.processor_count].max,
-        max_threads:     ThreadPoolExecutor::DEFAULT_MAX_POOL_SIZE,
-        # max_threads:     1000,
+    CachedThreadPool.new(
         auto_terminate:  opts.fetch(:auto_terminate, true),
-        idletime:        60, # 1 minute
-        max_queue:       0, # unlimited
         fallback_policy: :abort # shouldn't matter -- 0 max queue
     )
   end