make sure dasmap reload handler is assigned only once

* this is a rather rare case
use a parametrized singleton for DASMapping(config)

* 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 rewrite the __new__ method
  - this allows to keep the same interface as earlier, thus no changes are needed to other parts of code for now