kernel - Change the discrete mplock into mp_token

kernel - Change the discrete mplock into mp_token

* Use a lwkt_token for the mp_lock.  This consolidates our longer-term
  spinnable locks (the mplock and tokens) into just tokens, making it
  easier to solve performance issues.

* Some refactoring of the token code was needed to guarantee the ordering
  when acquiring and releasing the mp_token vs other tokens.

* The thread switch code, lwkt_switch(), is simplified by this change
  though not necessarily faster.

* Remove td_mpcount, mp_lock, and other related fields.

* Remove assertions related to td_mpcount and friends, generally folding
  them into similar assertions for tokens.