Run trans.close() at end of block if transaction already inactive

Run trans.close() at end of block if transaction already inactive

Modified the context manager used by :class:`_engine.Transaction` so that
an "already detached" warning is not emitted by the ending of the context
manager itself, if the transaction were already manually rolled back inside
the block. This applies to regular transactions, savepoint transactions,
and legacy "marker" transactions. A warning is still emitted if the
``.rollback()`` method is called explicitly more than once.

Fixes: #6155
Change-Id: Ib9f9d803bf377ec843d4a8a09da8ebef4b441665