Introduce separate entity object types for entity privileges

When granting or revoking a privilege on an entire entity, id 0 was used
to indicate the fact that we don't grant a privilege on a single object,
but on a whole entity. This caused confusion, because for entity USER,
for example, id 0 is a valid object id (user 'guest' uses it).
Any non-zero id dedicated to this cause obviously may be confused as well.
Fix this by creating separate schema_object_types for entities:
SC_ENTITY_SPACE, SC_ENTITY_SEQUENCE, etc.

Closes #3574
Needed for #3524