[disco] Fix memory leak in ServiceDiscoveryManager
The lambda we schedule in 25ms captures a strong reference to the
XMPPConnection. However the lambda is part of the scheduled action,
which we save in the renewEntityCapsScheduledAction field. This causes
a memory leak, since the ServiceDiscoveryManager now holds a strong
reference to its XMPPConnection.
Fix this by obtaining the strong reference to the XMPPConnection, if
one still exists, within the lambda.
