Use cache for database schema in get_schema()

[ligurio]

CRUD will use DDL schema to obtain a sharding key using get_schema(). However, on every get_schema() call DDL module scan spaces to build the current database schema. Need to cache a database schema.

[Totktonada]

Sadly, we have no public API for tracking a database schema version (without it there is no way to invalidate the cache, when some space or index is changed).

There is box.internal.schema_version(), which is obviously internal and has no guarantee that it'll exist in a future. It exists since 1.7.3-660-gcbfc235ee. We can take risks, use the internal function, file issue to expose it publicly (should be easy to implement and unlikely someone will be against). When it'll be exposed publicly, use the public function and fallback to the internal one if the public one does not exist.