Allow setting the size of the multiget pool at client initiation time.

It is safe to create multiple multiget pools because fetcher threads
aren't started until the pool is first used. Client objects that don't
modify the multiget_pool_size will use the predefined pool.
Add test for tweaking the multiget pool size.
Merge pull request #305 from basho/bugfix/gh305-override-multiget-pool-size

Ability to override multiget pool size

Reviewed-by: javajolt