Improve completion performance

[renkun-ken]

When I'm dealing with numerous large documents with numerous packages attached, the completion could become laggy as the first keystroke triggers the completion request which more than 2000 items are returned but most are useless.

I think we could truncate the completion list to a number and use isIncomplete = TRUE so that next keystroke could still trigger completion request but this time it narrows down the range often significantly. This would probably improve the performance of completion.