Drop default-related structures after the Table is dropped.

Drop default-related structures after the Table is dropped.

Fixed bug where a :class:`.Sequence` would be dropped explicitly before any
:class:`.Table` that refers to it, which breaks in the case when the
sequence is also involved in a server-side default for that table, when
using :meth:`.MetaData.drop_all`.   The step which processes sequences
to be dropped via non server-side column default functions is now invoked
after the table itself is dropped.

Change-Id: I185f2cc76d2011ad4dd3ba9bde5d8aef0ec335ae
Fixes: #4300