Integrate StateSync module with RemoveUntraceableHeaders option

[AnnaShaleva]

Current Behavior

StateSync module is not integrated with RemoveUntraceableHeaders option. I.e StateSync module will sync headers starting from genezis irrespective of RemoveUntraceableHeaders setting.

Expected Behavior

If RemoveUntraceableHeaders is enabled, then fetch only latest MaxTraceableBlocks number of headers prior to the current state sync point.

Possible Solution

#3789 introduces a part of it: it implements AddHeaders/HeaderHeight interfaces over StateSync module, which allows to use StateSync module instead of Blockchain as a backend for headers queue. What we still need to implement:

1. Add Module.headerHeight field that should be processed the same way as Module.blockHeight: set it to the latest desired header, update according to the headers insertion.
2. Request headers based on StateSync's header height instead of blockchain's header height.

In other words, we need to copy the solution that is used to request/insert blocks for/into the StateSync module and port this logic to headers-related parts. No new functionality id expected to be introduced.

Context

Depends on #3789.