api: add CRUD module support

[AnaNek]

This patch provides crud methods as request objects to support CRUD API.
The following methods are supported:

• insert
• insert_object
• insert_many
• insert_object_many
• get
• update
• delete
• replace
• replace_object
• replace_many
• replace_object_many
• upsert
• upsert_object
• upsert_many
• upsert_object_many
• select
• min
• max
• truncate
• len
• storage_info
• count
• stats
• unflatten_rows

Example:

    conn, err = Connect(server, opts)
    defer conn.Close()

    var conditions = []crud.Condition{
	{
		Operator: crud.LT,
		KeyName:  "id",
		KeyValue: uint(1020),
	},
    }

    var selectOpts = crud.SelectOpts{
	Timeout: crud.NewOptUint(timeout),
    }

    req := crud.NewSelectRequest(spaceName).
		Conditions(conditions).
		Opts(selectOpts)
	
    resp, err := conn.Do(req).Get()

Closes #108

[oleg-jukovec]

Thanks for the patch, I like this start.

[oleg-jukovec]

Thank you for the a lot of work done.

[DifferentialOrange]

Thank you for the updates! I think we'll be ready to merge this one after we clear up _many errors case

[DifferentialOrange]

Thank you for the updates and sorry for the long process! Approved from my side

[oleg-jukovec]

Could you please create an issue with missing functionality?

I think there should be at least:

1. Implement an ability to decode custom types via Result/ResultMany.

But maybe something else?