fix (rooms): Harden against race conditions.

Add locking to room init, turnover, and cancellation.
Add cleanup of nonexistent members in slot init.
Fix an issue where members were being charged for cancelling rooms.

bot/modules/accountability/TimeSlot.py

@@ -37,7 +37,7 @@ class SlotMember:
 
     @property
     def member(self):
-        return self.guild.get_member(self.data.userid)
+        return self.guild.get_member(self.userid)
 
     @property
     def has_attended(self):
@@ -254,6 +254,11 @@ class TimeSlot:
         Adds the TimeSlot to cache.
         Returns the (channelid, messageid).
         """
+        # Cleanup any non-existent members
+        for memid, mem in list(self.members.items()):
+            if not mem.data or not mem.member:
+                self.members.pop(memid)
+
         # Calculate overwrites
         overwrites = {
             mem.member: self._member_overwrite