minor fixes to prevent fast growth of dasmaps monitoring threads (to patch the release)

**background:** input_values_tracker and some other pieces in code (DBSPhedexService and LumiService, DBSDaemon's get_global_dbs_inst() & list_dbs_instances(), QLManager) was creating multiple new instances of DASMapping()
  - these instances of DASMapping don't get destroyed as described in #4146 (because the monitoring thread stores a reference to the object) and were contributing to increase dasmap reload messages in the logs

**changes:** this will prevent creating multiple instance of same DASMapping objects (as well as uneeded dasmap reload threads).
to keep changes in this patch to the minimum we override the ```__new__``` method to obtain a singleton which allows to keep the same interface as earlier, thus no changes are needed to other parts of code (at least for now).

---

fixes issue #4143 , and mostly fixes issue #4146

The solution can be tested by setting mappingdb.reload_time to a low interval. unpatched code would start reloading the maps more and more often. placing specific dasqueries was also increasing the "reloads", e.g. ```andrey``` triggers adjust_input which was using ```list_dbs_instances()```.

```
[mappingdb]
...
reload_time = 10
```

As a future solution (next release): I propose to get rid of these monitoring threads, and if needed initiate a reload from the highest level.