Problem: Completion side effects not working randomly.
Solution: When creating the table of LSP responses, the table index
was used, but this is not the same as the actual client_id, so it was changed
to use the client_id directly.
The `complete()` mechanism matches completion candidates against
the typed text, so strict pre-filtering isn't necessary.
This is a first step towards supporting postfix snippets (like
`items@insert` in luals)
