sql: replace affinity with field type in struct Expr

Also this patch resolves issue connected with wrong query plans during
select on spaces created from Lua: instead of index search in most cases
table scan was used. It appeared due to the fact that index was checked
on affinity compatibility with space format. So, if space is created
without affinity in format, indexes won't be used.
However, now all checks are related to field types, and as a result
query optimizer is able to choose correct index.

Closes #3886
Part of #3698