Avoid dreyfus_index_manager for _search_disk_size.
Avoid dreyfus_index_manager for _search_disk_size.

Previously, all use of the _search_disk_size endpoint was serialised
through gen_server calls in dreyfus_index_manager. However, the code
executed in response to the public interface function used no state from
the server, relying only on the passed arguments. Under load, this
resulted in dreyfus_index_manager accumulating a long message queue,
with the usual negative side effects.

cloudant-labs/dreyfus