Investigate internal usages of "isMaster" in sharding

Based on the comments in sharding_catalog_manager_shard_operations.cpp, addShard validates the host as a shard and calls "isMaster". Since we added the new hello command, we should evaluate whether or not we can change internal isMaster calls or if we need to rely on them to communicate with older versions of the server.

Related files:
sharding_network_connection_hook*
sharding_catalog_manager_add_shard_test
sharding_connection_hook*