sql: introduce vdbe_field_ref class

Refactored OP_Column instruction with a new vdbe_field_ref class.
The vdbe_field_ref is a reusable object that speeds-up field
access for given tuple or tuple data.
Introduced OP_Fetch opcode that uses vdbe_field_ref given as a
first argument. This opcode makes possible to perform binding
of a new tuple to an existent VDBE without decoding it's fields.

Needed for #3691