Fixes problem with idle connection reaping

The logic to reap idle connections from the RiakConnectionPool
was flawed and would hard loop under heavy load. This fixes that problem
and reduces the complexity by using a LinkedBlockingDeque to hold
connections.

Fixes #211
Added small optimization

There's no need to descend further once you find a connection
that does not need to be reaped since this is a LIFO.
Merge pull request #212 from basho/gh211-idle-reaper-bug

Fixes problem with idle connection reaping