add manual batch transactions

[kordano]

SUMMARY

Implements #403

Checks

Feature

• [x] Implements an existing feature request. Make sure the feature request has been accepted for implementation before opening a PR.
• [x] Related issues linked using fixes #number
• [x] Integration tests added
• [ ] Documentation added
• [x] Architecture Decision Record added
• [x] Formatting checked

ADDITIONAL INFORMATION

[whilo]

Just a minor remark, while sync? makes some sense as a flag, it will clash with the naming of the core.async flag we have now in konserve and might be confusing. Probably we should either fully namespace-qualify both or rename one.