Sync worker management: move logic out of companion object

[rfc2822]

Purpose

Currently, a lot of logic (managing one-time and periodic sync workers) happens in companion objects.

These objects are not ideal to use with DI and should rather contain constants and small helpers than real logic.

Short description

The logic of

• building and enqueing one-time sync workers,
• building and enabling/disabling periodic sync workers

is moved to a dedicated new class SyncWorkerBuilder which is usable with DI.

Checklist

• The PR has a proper title, description and label.
• I have self-reviewed the PR.
• I have added documentation to complex functions and functions that can be used by other modules.
• I have added reasonable tests or consciously decided to not add tests.

[ArnyminerZ]

Works great, and now everything makes more sense IMO. It will also make more sense to add the PUSH notification in SyncWorkerManager instead of where it is now.