Check for columns not part of mapping, correct mapping for eager_defaults

Check for columns not part of mapping, correct mapping for eager_defaults

Fixed two closely related bugs involving the mapper eager_defaults
flag in conjunction with single-table inheritance; one where the
eager defaults logic would inadvertently try to access a column
that's part of the mapper's "exclude_properties" list (used by
Declarative with single table inheritance) during the eager defaults
fetch, and the other where the full load of the row in order to
fetch the defaults would fail to use the correct inheriting mapper.

Fixes: #3908
Change-Id: Ie745174c917d512e2c46d9e3cc14512cde53cc9a