Make the state hash fully thread safe

Fixes issue described here: ruby-concurrency/concurrent-ruby#970

lib/finite_machine/hooks.rb

@@ -18,7 +18,7 @@ class Hooks
     def initialize
       @hooks_map = Concurrent::Map.new do |events_hash, hook_event|
         events_hash[hook_event] = Concurrent::Map.new do |state_hash, name|
-          state_hash[name] = []
+          state_hash.compute_if_absent(name) { [] }
         end
       end
     end