issue #594, handle SocketTimeoutException differently 

It consists of two changes:
* HThriftClient, generates HectorException in getCassandra
* Revert back the change for #434. I have tested the different scenario by shutting down Cassandra or make it responds really slow. I can't observe the problem described in #434. By coverting SocketTimeoutException to HTimedOutException, it avoids shutting down the pool when socket timed out.

The code has been in our production for over 2 weeks and everything works as expected.