Sec. index: make json array elements be encoded with depth information

[islamaliev]

Depth information for array elements allow to be more specific about which items you want to deal with.

At the moment we store all array element values as if they were just normal scalar fields on a json doc.

Lets say we have this schema:

type User {
  name: String 
  custom: JSON @index
}

And we create this documents:

{
  "name": "Andy",
  "custom": {
    "numbers": 4,
  }
}
{
  "name": "John",
  "custom": {
    "numbers": [3, 4],
  }
}

And then query:

query {
  User(filter: {custom: {numbers: {_none: {_eq: 4}}}}) {
    name
  }
}

This will result at the moment in an empty result, but correct behaviour would be to return "Andy" document as its number is not an array and shouldn't be checked against _none.

The indexes are stored roughly as follows:

/1/1/custom.number/3/john-doc-id
/1/1/custom.number/4/andy-doc-id
/1/1/custom.number/4/john-doc-id

CocroachDB uses arr suffix. With it the indexes might look like:

/1/1/custom.number/arr/3/john-doc-id
/1/1/custom.number/arr/4/john-doc-id
/1/1/custom.number/4/andy-doc-id