ScheduledMerges: rename MergePolicy to MergePolicyForLevel ...

... and rename its constructors from `MergePolicyTiering` and
`MergePolicyLevelling` to `LevelTiering` and `LevelLevelling` respectively. This
makes the prototype follow the naming in the real implementation, and it makes
it possible to introduce `MergePolicy` as a table-wide configuration option
without name clashes.

Author: jorisdral