- removed DEFAULT_MEASUREMENT global
- ACTIVE_MG is a proxy to the active measurement group
- ACTIVE_MG relies on 'get_active()' to get the active measurement group,
if no measurement group is defined an empty measurement group, without
counters, is created
- _active_name() method is split in two: get_active_name() and
set_active_name()
- the active measurement group name is always read from settings (redis)
- get_active_name() returns None if session doesn't have an active
measurement group