client context: Change API using GHashTable

Change:
milter_client_context_set_mail_transaction_shelf ->
milter_client_context_set_mail_transaction_shelf_value

milter_client_context_get_mail_transaction_shelf ->
milter_client_context_get_mail_transaction_shelf_value

Add:
milter_client_context_get_mail_transaction_shelf
binding ruby: Follow changes in previous commit