pico_service builtin user for internal inter-instance communication

Picodata follow-up

Todo:

• Mention proc_raft_promote in reference/proc_api.md, #192 (closed)

• Renew tutorial/access_control to mention pico_service user

• Follow-up for picodata!855 (merged)

• See also picodata#510 (closed)

• See also #188 (closed)