[Warlock] Demonology 12.1.0 pet bugs and cleanup reschedule

[millanzarreta]

Some minor Demonology Warlock changes, mainly focused on bug on 12.1.0 PTR:

• In PTR 12.1.0, without To Hell and Back, Implosion is bugged and only sends 5 Wild Imps to implode instead of 6 when 6 or more are active.
• Model a PTR 12.1.0 bugged interaction where a Wild Imp's final Fel Firebolt cannot trigger Infernal Rapidity if that same Wild Imp triggers Isolated Implosion after consuming all of its energy with that last Fel Firebolt.
• Vilefiend (all of its variants) now benefit from the Demonic Brutality talent on PTR 12.1.0. They remain bugged on 12.0.7, where they do not benefit from that talent.
• Replace direct reschedule_time manipulation when extending Dreadstalkers with the reschedule() event function.