Do away with company-completion-finished-hook.

Use post-completion command in the backend instead.
Also insert one space after the completion.
Fetch candidates asynchronously.
Backend rename and completion context.

- Check if slime is connected before trying to complete.
- Do no complete in comments or strings.
- Rename backend from `slime-company-backend' to `company-slime' for
  consistency.
Customization variables.

Whether to insert a space after completion, or if to complete in strings
and comments is now adjustable via custom variables.

M-x customize-group RET slime-company RET