BUG/MEDIUM: h2/threads: never release the task outside of the task handler

BUG/MEDIUM: h2/threads: never release the task outside of the task handler

Currently, h2_release() will release all resources assigned to the h2
connection, including the timeout task if any. But since the multi-threaded
scheduler, the timeout task could very well be queued in the thread-local
list of running tasks without any way to remove it, so task_delete() will
have no effect and task_free() will cause this undefined object to be
dereferenced.

In order to prevent this from happening, we never release the task in
h2_release(), instead we wake it up after marking its context NULL so that
the task handler can release the task.

Future improvements could consist in modifying the scheduler so that a
task_wakeup() has to be done on any task having to be killed, letting
the scheduler take care of it.

This fix must be backported to 1.8. This bug was apparently not reported
so far.