Make cache and values fully thread-safe

Not locking the default initialization can lead to race-conditions.

ref: ruby-concurrency/concurrent-ruby#970
Co-authored-by: Maciej Mensfeld <maciej@mensfeld.pl>
resolves puppetlabs#8951

lib/puppet/settings.rb

@@ -146,8 +146,8 @@ def initialize
     @configuration_file = nil
 
     # And keep a per-environment cache
-    @cache = Concurrent::Hash.new { |hash, key| hash[key] = Concurrent::Hash.new }
-    @values = Concurrent::Hash.new { |hash, key| hash[key] = Concurrent::Hash.new }
+    @cache = Concurrent::Hash.new { |hash, key| hash.compute_if_absent(key) { Concurrent::Hash.new } }
+    @values = Concurrent::Hash.new { |hash, key| hash.compute_if_absent(key) { Concurrent::Hash.new } }
 
     # The list of sections we've used.
     @used = []