Made HystrixTimer initialization thread-safe

The current initialization in `HystrixTimer` is not thread-safe: one thread may create the `ScheduledExecutor`, but a second thread may try to use it before it has been initialized, resulting in a `NullPointerException`. This race is easy to replicate by starting multiple Threads that each run a HystrixCommand simultaneously.

This pull fixes this by looping on the appropriate initialization condition.