Add pool timeout test for CPP-167

RE: fix in commit 43c68da645afa030dd210a1f3816640ad96e2947

This required some core changes to make the driver start with zero
connections. I pursued this approach in order to make the test
deterministic (as opposed to really trying to overload a connection, or
messing with timeouts).
Merge pull request #71 from datastax/CPP-167

Add pool timeout test for CPP-167