alter: move format compatibility check to space_check_format

We assume that if the new format can store tuples matching the old
format, we can update the space format without calling the engine
check_format callback. This is true for both memtx and vinyl but
not for memcs (EE), which doesn't support extending field types
(e.g. changing int16 to int32).

Let's call the engine check_format callback unconditionally and let
it decide whether tuple format checking can be skipped.

Needed for tarantool/tarantool-ee#694

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring