Add strategy that will optimize numeric aggregations

[To-om]

The PR #2200 add a strategy that asks the mixed index to count the number of matching elements. This optimization can be improved and add aggregation functions on numeric fields (min(), max(), mean() and sum()).

The strategy can optimize queries that look like:

• g.V().has(...).values('num_field').min()
• g.V().has(...).values('num_field').max()
• g.V().has(...).values('num_field').avg()
• g.V().has(...).values('num_field').sum()

The storage backend is then not touched and the performance should be highly increased.