Adapt to new v8::Persistent API, support V8 >= 3.21.12
Use v8's CopyablePersistentTraits for v8 >= 3.22.0
Remove Locker from timer thread
Merge pull request #43 from stesie/wip-ftbfs-v8-3.22

Adopt V8 API changes (up to 3.22.6)