timers: fixing API refs to use safe internal refs

Added safe internal references for 'clearTimeout(..)', 'active(..)', and
'unenroll(..)'. Changed various API refs from 'export.*' to use these
safe internal references.

Now, overwriting the global API identifiers does not create potential
breakage and/or race conditions. See Issue #2493.

PR-URL: https://github.com/nodejs/node/pull/5882


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Fixes: https://github.com/nodejs/node/issues/2493