[TACHYON-163]After 5 attempts we exit to avoid infinity retires

https://tachyon.atlassian.net/browse/TACHYON-163

Probably we should exit after attempts
If the connect method leads to infinity loop, user might think TahcyonWorker is fine with seeing the JVM process still existing.
Here is what I think: in function `register` only `mMasterClient.worker_register` can throw an IOException. In `MasterClient.worker_register` only `connect()` can throw an IOException. In `MasterClient.connect()` we throw an IOException because either `mIsShutdown` is true or the retry number is greater than 5.
So, if the on of the two conditions is satisfied, the IOException will be throwed all the way to `register` here. Then we should catch it and exit the JVM.