Ability to override multiget pool size

As far as multiget operation is concerned, I observed that the POOL_SIZE is calculated based on [`multiprocessing.cpu_count()`](https://github.com/basho/riak-python-client/blob/master/riak/client/multiget.py#L30). 

This may seem optimal from the client aspect but may more easily lead to overload situations, whereas the load can be adjusted if you diminish the number of parallel threads, thus sacrificing some speed for stability.

Since [RIAK_MULTIGET_POOL](https://github.com/basho/riak-python-client/blob/master/riak/client/multiget.py#L152) is already initialized, can someone propose a way (if possible) of  overriding the POOL_SIZE parameter somehow?