multi: split multi_runsingle into sub functions

Introduce five functions named after the state they serve:

- state_connect for MSTATE_CONNECT
- state_do for MSTATE_DO
- state_performing for MSTATE_PERFORMING
- state_ratelimiting for MSTATE_RATELIMITING
- state_resolving for MSTATE_RESOLVING

Closes curl#15418

Author: bagder