Test + fix for epoll and kqueue selector modules to properly unregister

event handles that have the key type "User"

tests/async/testmanyasyncevents.nim

@@ -0,0 +1,23 @@
+discard """
+output: '''
+hasPendingOperations: false
+triggerCount: 512 
+'''
+"""
+
+import asyncDispatch
+
+var triggerCount = 0
+var evs = newSeq[AsyncEvent]()
+
+for i in 0 ..< 512: # has to be lower than the typical physical fd limit
+  var ev = newAsyncEvent()
+  evs.add(ev)
+  addEvent(ev, proc(fd: AsyncFD): bool {.gcsafe,closure.} = triggerCount += 1; true)
+
+for ev in evs:
+  ev.trigger()
+
+drain()
+echo "hasPendingOperations: ", hasPendingOperations()
+echo "triggerCount: ", triggerCount