Don't leak pending call objects when peer disconnects

Unlike in regular connection to DBus server, we don't get pending call
notifies when a peer drops the connection in peer-to-peer mode.

Thus, we need to keep track of pending calls in such cases and get rid of
them in ~QDBusConnectionPrivate().

Change-Id: I83e20db0bc7b2ebf509c7fdb1382ffc7d0ede9d3
Done-with: Kalle Vahlman <kalle.vahlman@movial.com>
Reviewed-by: Daniele E. Domenichelli <daniele.domenichelli@gmail.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>