Fix | Fix SharedTimer implementation to use class level lock for thread safety

[cheenamalhotra]

Fixes #1040

[sehrope]

The static isRunning() method should be updated to use the explicit lock object as well.

[sehrope]

Actually maybe it doesn't need the explicit synchronized lock at all as it's accessing the volatile field.

The method level synchronized can be removed though as the only thing it would be synchronizing is concurrent callers to that one method.