Simplify fork timeouts in sensu client

It looks like the sensu clients are now hanging on `IO.select` in the Riak boxes -- and the processes are just in a futex_wait mode and not timing out.  As a result, I'm simplifying the whole thing.

In this change, we're going to see how just relying on the timeout behaves.  In this case, we'll simplify start a thread which waits for the timeout period and kills off the process if it completes.  As a result, I expect this technique to be more reliable since it has no dependencies on any external i/o pipes.

We'll have to see how this behaves in production -- it's incredibly hard to reproduce locally.

@deanxorr