fix(locker): recouple Enter/Exit into Locker with correct ordering

The pure-mutex decoupling (cac04fa, a969248) was a workaround for a
race condition: Enter() was called before acquiring the lock, modifying
V8's entry_stack_ (not thread-safe) without synchronization.

The real fix is to swap the order:
- new:  Lock then Enter (hold lock before touching entry_stack_)
- drop: Exit then Unlock (exit while still holding the lock)

Author: max-lt