* Create a `SyncStatus` helper type
Keeps track if the synchronizer is close to the chain tip or not.
* Refactor `ChainSync` ctor. to return `SyncStatus`
Change the constructor API so that it returns a higher level construct.
* Test if `SyncStatus` waits for the chain tip
Test if waiting for the chain tip to be reached correctly finishes when
the chain tip is reached. This is done by sending recent sync lengths to
the `SyncStatus` instance, and checking that every time a separate
`SyncStatus` instance determines it has reached the tip the original
instance wakes up.
* Add a temporary attribute to allow dead code
The code added isn't used yet, so we'll add a temporary waiver until
another PR is merged to use them.