Don't use batch size for channel buffers

Channel buffers are there to let workers do a bit more work
while things are running, but batch size is for the amount of stuff
we want to accumulate before dropping a whole batch of computed
results off with the server. There's no reason for the channel buffer
to be that size, and having it be that size (which is typically at
least 65536) means that each import worker is typically sitting on
a channel that has about a million interface-flavored objects containing
pointers to records in it, and suddenly we have memory pressure.

import_manager.go

@@ -39,7 +39,7 @@ func (rim recordImportManager) run(field *Field, iterator RecordIterator, option
 	}
 
 	for i := range recordChans {
-		recordChans[i] = make(chan []Record, options.batchSize)
+		recordChans[i] = make(chan []Record, 16)
 		recordBufs[i] = make([]Record, 0, 16)
 		chans := importWorkerChannels{
 			records: recordChans[i],