MINOR: check: remember when we migrate a check

MINOR: check: remember when we migrate a check

The goal here is to explicitly mark that a check was migrated so that
we don't do it again. This will allow us to perform other actions on
the target thread while still knowing that we don't want to be migrated
again. The new READY bit combine with SLEEPING to form 4 possible states:

 SLP  RDY   State      Description
  0    0    -          (reserved)
  0    1    RUNNING    Check is bound to current thread and running
  1    0    SLEEPING   Check is sleeping, not bound to a thread
  1    1    MIGRATING  Check is migrating to another thread

Thus we set READY upon migration, and check for it before migrating, this
is sufficient to prevent a second migration. To make things a bit clearer,
the SLEEPING bit was switched with FASTINTER so that SLEEPING and READY are
adjacent.