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

* input_values_tracker was creating a new instance of DASMapping() on every maps reload
  - these instances of DASMapping don't get destroyed as described in #4146 (because the monitoring thread stores a reference to the object)
  - so instead of create a new DASMapping object, we pass one (even if that would lower the number of parameters to init_trackers)
  - P.S. the main impact of this issue was increased number of threads, putting some additional work to the server (reloading maps), appart from additional load this is shall not have impacted the functionallity, as these long-living DASMapping objects were not used after the first access
* still, the number of DASMapping objects would continue to grow on every MongoDB restart
  - this is because DASCore is being recreated in DASWebSrv. #4146
  - this is out of this patch because of comparatively small impact, and large number of changes needed to DAS. 

fixes issue #4143

partially fixes issue #4146

As a future solution (next release): I propose to get rid of these multiple monitoring threads, and if needed initiate the reload from the highest level