Update ConnectionPoolImpl.java

[Aatish04]

Not able to connect to dynomite if first host is down because even if another host returns the results we are not resetting "lastException" variable, we end up throwing exception.

[jcacciatore]

I recommend including a unit test for point fixes such as this.

Out of scope for this change, however, a more robust way to implement this would be to rely only on RetryPolicy instead of a special purpose variable. Unfortunately RetryPolicy does not support retrieval of the last failure (currently you can only mark that a failure occurred and get the attempts) but we should change this to facilitate getting rid of the lastException variable altogether.

[ipapapa]

@Aatish04 please see @jcacciatore feedback above if you want us to proceed your PR.