getting the bg task production ready

[Trolldemorted]

Remaining todos:

• fix the deadlock issues f9fc084
• fix we aren't reloading the selected conversation properly when resuming #125, it causes crashes when new messages arrive because the indices are messed up c28b03b
• fix App won't start because bg task setup fails ("not enough quotas available") #130, it doesn't crash your app but I recon it disables the bg task
• fix bg task does not release the mutex on its own #134, windows takes a while to free mutexes of dead processes e124758
• abort the bg task on app startup 0e11a8e

[Trolldemorted]

Everything done except #130!

cc @golf1052