sql: get rid off tnum field of struct Table

Basic structures (struct Table/Index) of legacy SQL's data
dictionary used to so-called tnum to refer to engine's
btree structures. Tarantool used this field to store composition
of space_id and index_id. Recently both structures incorporated
native space_def/index_def descriptors.
This patch finally removes tnum field. It also refactors
init_callback machinery, removing varargs from declarations.

Closes #3482