The aim of this ticket is to improve and clean-up the ShardRegistry API. In particular will address the following points:

• Change ShardRegistry::getAllShardIds to return result by using the return value instead of using modifying the received std::vector pointer (C style)
• The same for ShardRegistryData::getAllShardIds
• In ShardRegistryData::getAllShardIds remove the check for the config server since we are now storing config server shard in a separate data structure.
• In ShardRegistryData::getAllShardIds use faster std::vector instead of an std::set. In fact at this level we don't care about ordering the result and moreover we are sure no duplicates will be returned.
• Update function comments to better describe their behaviors.