disabled 'disk_size' in Request object evsukov89@couchbase-lite-ios#1

because on dbs with large number of attachments it's very slow due
the NSFileManager getting attributes for all attachments blobs
stored on a disk
Response object for show funcs evsukov89@couchbase-lite-ios#3
Response object for list funcs evsukov89@couchbase-lite-ios#3