timers: use consistent checks for canceled timers

Previously not all codepaths set `timer._idleTimeout = -1` for canceled
or closed timers, and not all codepaths checked it either.

Unenroll uses this to say that a timer is indeed closed and it is the
closest thing there is to an authoritative source for this.

Refs: https://github.com/nodejs/node/pull/9606
Fixes: https://github.com/nodejs/node/issues/9561
PR-URL: https://github.com/nodejs/node/pull/9685


Reviewed-By: Rich Trott <rtrott@gmail.com>