Gmoshkin/service auth

Summary

• fix: used to not reconnect after some tarantool errors

• test: clean up a bunch of tests

• fix: use user 'pico_service' for inter-instance communication

• refactor: pico.raft_timeout_now -> .proc_raft_promote

• fix: used to panic when checking login attempts via underprivileged user

• feat: grant 'public' access to .proc_* with pub visibility

• feat: introduce pico_service builtin user

• refactor: on_master_create_user now supports creating a user with no permissions

• chore: acl related doc comments

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• Blocked by tarantool-module!467 (merged)
• Close #510 (closed)
• Target branch: master
• Docs follow-up: docs#181 (closed)