Do not auto start/shutdown ordering and server managers at module import

[manuelma]

Re Spine-project/Spine-Database-API#187

...but provide functions to do it on demand

(description of changes)

Fixes #187

Checklist before merging

• [x] Documentation (also in Toolbox repo) is up-to-date
• [x] Release notes in Toolbox repo have been updated
• [x] Unit tests have been added/updated accordingly
• [x] Code has been formatted by black
• [x] Unit tests pass

[codecov-commenter]

Codecov Report

Base: 74.46% // Head: 74.32% // Decreases project coverage by -0.14% :warning:

Coverage data is based on head (a73cb10) compared to base (ba43f01). Patch coverage: 29.72% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #194 +/- ## ========================================== - Coverage 74.46% 74.32% -0.15% ========================================== Files 71 71 Lines 11696 11708 +12 Branches 2145 2145 ========================================== - Hits 8710 8702 -8 - Misses 2536 2560 +24 + Partials 450 446 -4 ``` | [Impacted Files](https://codecov.io/gh/Spine-project/Spine-Database-API/pull/194?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Spine-project) | Coverage Δ | | |---|---|---| | [spinedb\_api/spine\_db\_server.py](https://codecov.io/gh/Spine-project/Spine-Database-API/pull/194/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Spine-project#diff-c3BpbmVkYl9hcGkvc3BpbmVfZGJfc2VydmVyLnB5) | `34.26% <27.27%> (-3.90%)` | :arrow_down: | | [spinedb\_api/db\_mapping\_base.py](https://codecov.io/gh/Spine-project/Spine-Database-API/pull/194/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Spine-project#diff-c3BpbmVkYl9hcGkvZGJfbWFwcGluZ19iYXNlLnB5) | `83.74% <50.00%> (+0.43%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Spine-project). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Spine-project)

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