Use a Concurrent::Array for storing messages in the Buffer

This ensures that we can still concurrently add messages without messing
with the internal Array.

lib/rackstash/buffer.rb

@@ -128,7 +128,7 @@ module Rackstash
     #
     # @return [self]
     def clear
-      @messages = []
+      @messages = Concurrent::Array.new
       @fields = Rackstash::Fields::Hash.new(forbidden_keys: FORBIDDEN_FIELDS)
       @tags = Rackstash::Fields::Tags.new
       @timestamp = nil