srml: system: prune block hash mapping (#3062)

* srml: system: prune block number to hash mapping

* srml: system: add test for block hash mapping pruning

* node: bump runtime version

* srml: system: wrap long line

* srml: system: use parameter type for block hash count

* srml: system: prune block hash mapping before storage root calculation

* srml: system: keep the genesis hash in block number map