alter: fix wrong usage of BOX_CLUSTER_FIELD_UUID

It was used for getting _schema tuple[1] field. Worked because
BOX_CLUSTER_FIELD_UUID is also 1. This commit introduces
BOX_SCHEMA_FIELD_VALUE.

It is going to be used in the next commits more extensively for
new _schema tuples.

Part of #5029

NO_DOC=refactoring
NO_CHANGELOG=refactoring
NO_TEST=already covered