Add thread_safe for handling the @children inside of Hermann::Producer

This isn't comprehensive, still debating a comprehensive approach to #50 that
I'm happy with

References #50

hermann.gemspec

@@ -25,6 +25,7 @@ Gem::Specification.new do |s|
 
   s.add_dependency 'concurrent-ruby', '~> 0.7.0'
   s.add_dependency "zk", "~> 1.9.4"
+  s.add_dependency 'thread_safe', '~> 0.3.4'
 
   if RUBY_PLATFORM == "java"
     #IMPORTANT: make sure that jar-dependencies is only a development dependency of your gem. if it is a runtime dependencies the require_jars file will be overwritten during installation.

lib/hermann/producer.rb

@@ -1,7 +1,8 @@
+require 'thread_safe'
+
 require 'hermann'
 require 'hermann/result'
 
-
 if RUBY_PLATFORM == "java"
   require 'hermann/provider/java_producer'
 else
@@ -26,7 +27,7 @@ module Hermann
       end
       # We're tracking children so we can make sure that at Producer exit we
       # make a reasonable attempt to clean up outstanding result objects
-      @children = []
+      @children = ThreadSafe::Array.new
     end
 
     # @return [Boolean] True if our underlying producer object thinks it's