Fix editing a locked db

[PiispaH]

In a locked db if existing values are updated, the changes are stored in the cache and can be committed when the db is no longer locked. Removing items also works the same way. Adding items to a locked db is denied and will result in an error. If trying to commit changes in the db while it is locked an error telling the commit failed pops up. When the lock is resolved the changes can again be committed.

Re spine-tools/Spine-Toolbox#2201

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

Patch coverage: 44.82% and project coverage change: -0.09% :warning:

Comparison is base (bd337e8) 75.43% compared to head (189eac0) 75.34%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #258 +/- ## ========================================== - Coverage 75.43% 75.34% -0.09% ========================================== Files 76 76 Lines 10921 10941 +20 Branches 2378 2378 ========================================== + Hits 8238 8244 +6 - Misses 2243 2257 +14 Partials 440 440 ``` | [Files Changed](https://app.codecov.io/gh/spine-tools/Spine-Database-API/pull/258?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spine-tools) | Coverage Δ | | |---|---|---| | [spinedb\_api/db\_mapping\_add\_mixin.py](https://app.codecov.io/gh/spine-tools/Spine-Database-API/pull/258?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spine-tools#diff-c3BpbmVkYl9hcGkvZGJfbWFwcGluZ19hZGRfbWl4aW4ucHk=) | `77.35% <27.27%> (-1.58%)` | :arrow_down: | | [spinedb\_api/db\_mapping\_remove\_mixin.py](https://app.codecov.io/gh/spine-tools/Spine-Database-API/pull/258?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spine-tools#diff-c3BpbmVkYl9hcGkvZGJfbWFwcGluZ19yZW1vdmVfbWl4aW4ucHk=) | `95.53% <50.00%> (-1.07%)` | :arrow_down: | | [spinedb\_api/db\_mapping\_commit\_mixin.py](https://app.codecov.io/gh/spine-tools/Spine-Database-API/pull/258?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spine-tools#diff-c3BpbmVkYl9hcGkvZGJfbWFwcGluZ19jb21taXRfbWl4aW4ucHk=) | `69.23% <55.55%> (-4.11%)` | :arrow_down: | | [spinedb\_api/db\_mapping\_update\_mixin.py](https://app.codecov.io/gh/spine-tools/Spine-Database-API/pull/258?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spine-tools#diff-c3BpbmVkYl9hcGkvZGJfbWFwcGluZ191cGRhdGVfbWl4aW4ucHk=) | `78.60% <60.00%> (-0.61%)` | :arrow_down: |

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