Follow-up on #1419 (#1531)

* Parachains source cosmetic changes

- Make `ParaHashAtSource` more generic
- Modify `on_chain_parachain_header` to return `HeaderId`
- Shortening variable names

Signed-off-by: Serban Iorga <[email protected]>

* Change ParachainsSource::max_head_id type

Change ParachainsSource::max_head_id to Arc<Mutex<NoopOption>>

Signed-off-by: Serban Iorga <[email protected]>

* code review changes