Change cache ets table from set to bag

As the tables are created per-host-type, a table can have more than one
domain within, and so there's no unique key properly defined,
considering that the same namepart can be used in two different domains,
and the same domain contains many nameparts.

Restore it as a set, and actually fix the key inserted

src/mongoose_users.erl

@@ -86,7 +86,7 @@ remove_user(Acc, LUser, LServer) ->
     mongoose_hooks:simple_acc().
 remove_domain(Acc, HostType, Domain) ->
     Tab = tbl_name(HostType),
-    ets:match_delete(Tab, {Domain, '_'}),
+    ets:match_delete(Tab, {{Domain, '_'}}),
     Acc.
 
 %%====================================================================
@@ -107,7 +107,7 @@ does_cached_user_exist(HostType, LServer, LUser) ->
 put_user_into_cache(HostType, LServer, LUser) ->
     Key = key(LUser, LServer),
     Tab = tbl_name(HostType),
-    ets:insert(Tab, Key),
+    ets:insert(Tab, {Key}),
     ok.
 
 -spec delete_user(mongooseim:host_type(), jid:lserver(), jid:luser()) -> ok.