gs-metered: Count all references to task in block_scheduler_schedule_cb
In block_scheduler_schedule_cb, a GCancellable::cancelled signal
handler was connected to a tasks's cancellable, with a pointer to the
task itself as data. However, this was done without incrementing the
reference count for the task, resulting in an error due to the task
being prematurely freed when g_cancellable_disconnect was called.
In addition to resolving that, we will change block_check_cb to
increment the task's reference count while it is in use.
Closes #2516 (closed)
This is a backport of !1966 (merged).