fix: report a more specific error when plugin is dropped while we're waiting for enable plugin operation to finish

- Simplify code
- Check plugin exists on client side

- Move all preconditions checking to the client
- Return errors if preconditions fail instead of silently ignoring them

- Don't ignore operation if plugin is already enabled,
  the client must take care of this via the CaS ranges

- Don't propose mutliple Raft operations during one step as this may
  lead to inconsistencies in case of crashes

- Don't ignore operation if plugin is already installed,
  the client must take care of this via the CaS ranges

- Don't propose multiple Raft operations during one step as this may
  lead to inconsistencies in case of crashes