Rs map undefined fix

This branch fixes two intermittent bugs related to structural problems in ThResultSetModel. The fix required the following modifications: re-structure ThResultSetModel so that it manages resultsets for all repositories a user visits during a ui session and modify resultset paging, replacing the relative offset with the last resultset id received. An added benefit of these changes is data caching when toggling between watched repositories. This branch has a dependency on fix treeherder-service branch ``fix-resultset-paging``.