Synchronize FilterChain#to_s and FilterChain#inspect

This ensures that any access to the internal @filters array is only done
with an aquired lock to ensure data consistency.

lib/rackstash/filter_chain.rb

@@ -227,9 +227,11 @@ module Rackstash
 
     # @return [String] a string representation of `self`
     def inspect
+      synchronize do
         id_str = Object.instance_method(:to_s).bind(self).call[2..-2]
         "#<#{id_str} #{self}>"
       end
+    end
 
     # @return [Integer] the number of elements in `self`. May be zero.
     def length
@@ -259,7 +261,7 @@ module Rackstash
 
     # @return [String] an Array-compatible string representation of `self`
     def to_s
-      @filters.to_s
+     synchronize { @filters.to_s }
     end
 
     private