(MAINT) Add optional API Request Logging

This was partially an exercise to use middleware, but also to enable
basic logging for the API by logging all the API calls to the logger.

spec/unit/env_config.rb

@@ -30,6 +30,7 @@ describe 'Vmpooler' do
         ['experimental_features', test_bool, nil],
         ['purge_unconfigured_folders', test_bool, nil],
         ['usage_stats', test_bool, nil],
+        ['request_logger', test_bool, nil],
         ['extra_config', test_string, nil],
       ]