wg: Port #35: refactor noise keypair functions to avoid epoch

wg: Port #35: refactor noise keypair functions to avoid epoch

- Refactor the functions to avoid using epoch or deferred operations.
  So noise_keypair_put() is simplified and remove the obsolete
  noise_keypair_smr_free() function.
- Remove 'refcount_acquire_if_not_zero()' and use plain refcount instead,
  since there's no more deferred frees.
- Use shared locks instead of epoch sections; meanwhile, rename
  'r_keypair_mtx' to 'r_keypair_lock' because it now supports both
  shared and exclusive locks.