Clean up sentinel initialization code

- Create wp_get_txn(void) function
  - Wrap static local txn initialization, remove it from wp_get_txn_head
    function call
- Replace body of wp_get_txn_head with call to wp_get_txn only if the
  wp_txn_head static local is NULL
- Rename wp_memory_txn_t typedef to wp_memory_txn
- Add additional restrict type modifiers where appropriate