Adding a few integration points to the C++ TSocketPool.

Adding a few integration points to the C++ TSocketPool.

- Adding ability to use a vector of TSocketPoolServers to construct a TSocketPool
- Ability to get back the list of TSocketPoolServers

This is especially useful in multithreaded client code that
will just keep around the list of servers, and create the pool
on every request. Since TSocketPool updates the failure stuff,
we need a way to get back the updated TSocketPoolServers

Reviewed By: aditya

Test Plan: just compiling the code

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@665537 13f79535-47bb-0310-9956-ffa450edef68