Remove synchronous timeout

[mattrjacobs]

This removes the synchronous timeout path entirely and relies solely on async timeouts coming from the HystrixTimer thread. Previously, calling HystrixCommand.execute() or .queue() would trigger the synchronous timeout.

Internal performance tests have indicated that performance is generally unchanged with this PR. I've also created a issue (#578) to track getting performance tests included as part of this project.

The benefits of this change are:

1. Simplification
2. Fixes Timeout mechanism doesn't work for command which is queued but not blocked upon #514
3. Removes the code-path shown in the stack trace of Deadlock under "Fallback: Cache via Network" pattern #352 - so we know that there's at least one path through the synchronous timeout flow that's not thoroughly tested. Removing the synchronous timeout path entirely may fix this problem.

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