Revert "Prevent multiple instances of the same DownloadInfo pointer being added to the cancellation queue"

This reverts commit eb78c7bda7c1859831997a54a3e3eadbb0993dd7.
Revert "Fix MythDownloadManger after 997f7aa10. It was possible for queue and cancel requests to be handled out of order resulting in newly queued downloads being cancelled almost immediately. We avoid this by removing any cancelled downloads  from the download queue to a new cancellation queue, then we clean up the cancelled downloads later without affecting new additions."

This reverts commit 277feb598e975d43c68f4b0a042a3ce6d5fd48bb.
Ensure cancelling a download is done in the same thread it started; take #2

MythDownloadManager::cancelDownload() needs to wait for the cancel to actually occur as some code relies on that behaviour.
This includes the same fix as commit eb78c7bda7c1859831997a54a3e3eadbb0993dd7 "Prevent multiple instances of the same DownloadInfo"

Fixes #11663
Fix potential deadlocks.

when changing channel very quickly, and depending on when a cancel would be called, we could enter various deadlocks.
Do not use QMap::operator[] unless map definitely contains the key

QMap::operator[] would add here a NULL entry in the list of cards which would later cause a null dereference.

Fixes #11279
Fix comment typo