timers: minor _unrefActive fixes and improvements

This commit addresses most of the review comments in
https://github.com/nodejs/node/pull/2540, which are kept in this
separate commit so as to better preserve the prior two patches as they
landed in 0.12.

This commit:
- Fixes a bug with unrefActive timers and disposed domains.
- Fixes a bug with unrolling an unrefActive timer from another.
- Adds a test for both above bugs.
- Improves check logic, making it stricter, simpler, or both.
- Optimizes nicer with a smaller, separate function for the try/catch.

Fixes: https://github.com/nodejs/node-convergence-archive/issues/23
Ref: https://github.com/nodejs/node/issues/268
PR-URL: https://github.com/nodejs/node/pull/2540


Reviewed-By: bnoordhuis - Ben Noordhuis <info@bnoordhuis.nl>