api: add pagination support

[oleg-jukovec]

A user could fetch a position of a last tuple using a new method of the SelectRequest type:

selectRequest = selectRequest.FetchPos(true)

The position will be stored in a new field of the Response type:

Response.Pos

A user could specify a tuple from which selection must continue or its position with a new method of the SelectRequest type:

selectRequest = selectRequest.After([]interface{}{23})

or

selectRequest = selectRequest.After(resp.Pos)

In action it looks like:

req := NewSelectRequest(space).Key(key).Limit(10).FetchPos(true)
for condition {
    resp, err := conn.Do(req).Get()
    // ...
    req = req.After(resp.Pos)
}

1. Introduce pagination tarantool#7639
   I didn't forget about (remove if it is not applicable):

• Tests (see documentation for a testing package)
• Changelog (see documentation for changelog format)
• Documentation (see documentation for documentation style guide)

Related issues:

Closes #246

[DifferentialOrange]

Thank you for your PR! Seems nice, I've found a couple of tricks that may be useful for me later. I have left a several minor comments.