Implement properties fetching mode to retrieve all necessary properties via a single slice query but reducing the Slice query scope to the most left and most right property keys

[porunov]

In #3803 we are adding 2 modes of preFetching properties:
all_properties - retrieves all properties via a single slice query.
required_properties_only - retrieves only requested properties but via separate slice query per each property.

This is a trade off a user needs to choose right now. Either using multiple slice queries (probably worse performance for cases when there are not too much properties or they are small) or use a single slice query but fetch all vertex properties which might fetch too much redundant data in some cases.
Here I explained why it's not possible to have the best from both worlds right now (i.e. why it's not possible to retrieve only requested properties via a single slice query yet).
That said, it seems we can implement a slightly smarter mode which always uses a single slice query and fetches all properties from the most left requested property to the most right requested property (sorted by key ids, of course).

For example, let say the vertex have the next properties in the row:
prop1, prop2, prop3, prop4, prop5, prop6, prop7, prop8.

Let's say that the user sends the next Gremlin query: g.V(vertexId).valueMap("prop4", "prop6", "prop2"). Now if we try to find the most left property and the most right property from the requested list, we are getting:
"prop2" - the most left key (or the key with the smallest id).
"prop6" - the most right key (or the key with the highest id).

In this situation we definitely know that the user isn't interested in any properties which might come before prop2 and we definitely know that the user isn't interested in any properties which might come after prop6. Thus, our single slice query can be reduced to retrieve all properties between prop2 and prop6. I.e., the the case with g.V(vertexId).valueMap("prop4", "prop6", "prop2") we will retrieve properties prop2, prop3, prop4, prop5, prop6, but we will not retrieve properties prop1, prop7, prop8.
Of course, in the worst case, when the user sends a request like g.V(vertexId).valueMap("prop1", "prop8") - we will need to retrieve ALL properties if we want to use just a single slice query.
This technique won't improve worst case scenarios, but it might well improve average case scenarios.

@li-boxuan Since you had a good dive into the storage layer data-structure, I wander if you have any thoughts about this idea or weather you thought about it before.
cc @FlorianHockmann since you had related question here.

[li-boxuan]

Sounds like a good plan!