Wrap global wp_txn_head in a static function

- Move wp_txn_head into a function with designated initializer
- Replace all references to wp_txn_head with wp_get_txn_head()
- Re-order struct wp_memory_txn
- Correct description of wp_txn_head throughout
- Small attempt to pretty-print wp_get_txn_head. It's ugly
- Remove now-useless code to free(wp_txn_head)
- Remove most initialization code from wp_memory_initialize() function
  as it's now wrapped in wp_get_txn_head()