Call QObject::disconnectNotify() when receiver is destroyed

Call QObject::disconnectNotify() when receiver is destroyed

Store the signal index in QObjectPrivate::Connection, thereby making
it available in "implicit" disconnect contexts (i.e., receiver
deletion).

This change does not cause the size of QObjectPrivate::Connection
to grow (still 40 bytes on 32-bit Linux, 72 bytes on 64-bit Mac).

Valgrinding the new benchmark indicates that the percentage of the
time spent in the QObject destructor increased from 7.8% to 8.4%
on ia32, for that particular stress test; the increase is the
combined cost of calling metaObject(), QMetaObjectPrivate::signal(),
and disconnectNotify() for one connection. In practice, the measured
wallclock time increased by about 3ms for a 500ms run (which
repeatedly constructs, connects, and destroys an object).

Task-number: QTBUG-4844

Change-Id: I1beb01c753f31542fc0acb62edb4c6d165fcc5b4
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>