Recalculate maxQueueSize based on shards count

[dkropachev]

Currently maxQueueSize is 256 by default. Problem is that some hosts can have 1 shard, others can have 256 shards. While having cap of 256 pending requests for 1 shard host is ok, for bigger host of 256 shards it is way to low. This commit scales maxQueueSize by number of shards, which allow default value works for hosts of any size.

Fix: https://github.com/scylladb/java-driver/issues/577