deferred: don't delete self, to allow smart pointers to manage

[jkarneges]

The Deferred class provides a generic interface for an async operation, signaling a QVariant result. Currently it deletes itself upon completion (literally with delete this), intended to enable a fire-and-forget developer experience when used along with QObject parenting. The idea is whether it completes or its parent goes away, it is automatically cleaned up, without the caller needing to explicitly track it. However, we are trying to avoid QObject parenting, preferring to use smart pointers instead, and self-destruction is incompatible with smart pointers.

In order to allow Deferred instances to be managed with smart pointers, this PR removes the self destruction and updates all the places Deferred is used to ensure instances are destroyed on completion or when the parent/owner goes away. In some places we were already using smart pointers to manage some instances, which was almost certainly wrong (possibly leading to double-deletions) and this PR fixes those cases too.

The resulting developer experience is not as nice but at least this fixes some wrongness and helps get us off QObject.

[jkarneges]

Technically this should be here since Deferred inherits from QObject, but probably not for much longer.