When returning pending queries `Deferred`, a caller could add its own callbacks/errbacks
and modify the result. When another caller was reusing the `Deferred`, it got the modified
result (which could be `None`) instead of the expected `DiscoInfo`.
This has been fixed by creating and new `Deferred` on each request, and chaining it to the
original pending queries using the new `fork_deferred` helper method, so each caller get
its own isolated `Deferred`.
In `fork_deferred`, `cancel` method is used to call the parent Deferred `cancel`, to avoid
a wrong Exception propagation and the chained Deferred to be eventually called twice
(triggering `AlreadyCalledError`).
`cleanup_query` has been moved above `infos_cb` callback, as it should be the first
callback called.
2c5713b486