Consolidate feed_params and channel_index in config

[adamcfraser]

Configuration of the channel index will be more straightforward if we move all configuration into the channel_index section of the config. Currently the feed_params are only relevant for distributed index writers.

We should move the feed_params.num_shards property under the channel_index section of the config, as it's only relevant to channel_index writers.

[tleyden]

@sethrosetter just a reminder that you guys will need to update your configs once this change is made.