Fixed performance issue introduced in v0.0.11

Reverted to installing DebugEventListener only after a SIGHUP is
received.  This requires us to enter the default node isolate,
since `SetDebugEventListener[2]` functions can be called only from
inside a valid v8 isolate.  Unfortunately, this likely means
that an infinite loop will no longer be able to be broken into
reliably.

In addition, to restore performance, we reset the listeners to
NULL after (optionally) printing the stacktrace.
Yes, you're right that it can be hoisted since obj does not change.  It may help performance a small amount (I didn't write this code originally), but this code should only be executed when a process is not responding well (i.e. when monitr sends a SIGHUP event), so it's unlikely that performance is the primary concern at this point.