Topics' schema change: number -> integer for timestamp values

### Description of Technical Debt

Not a big deal, but the 'integration layer' like this should have correct schema. Timestamps should not allow floating point numbers to get into the system, it should have correct data type setup - and that is `integer`, not `number`.

### Impact of Technical Debt

Increases bug risks & allows invalid data to get into the system.

### Category

Code Quality / Refactoring

### Priority

Medium - Should be addressed soon

### Proposed Solution

Go over all timestamp fields in all the schemas and make sure that the type is `integer` for these, not `number`.