core: make ExecRuntime be manager managed object

Before this, each ExecRuntime object is owned by a unit. However,
it may be shared with other units which enable JoinsNamespaceOf=.
Thus, by the serialization/deserialization process, its sharing
information, more specifically, reference counter is lost, and
causes issue #7790.

This makes ExecRuntime objects be managed by manager, and changes
the serialization/deserialization process.

Fixes #7790.