MEDIUM: stats: introduce new actions to simplify admin status management

MEDIUM: stats: introduce new actions to simplify admin status management

Instead of enabling/disabling maintenance mode and drain mode separately
using 4 actions, we now offer 3 simplified actions :
  - set state to READY
  - set state to DRAIN
  - set state to MAINT

They have the benefit of reporting the same state as displayed on the page,
and of doing the double-switch atomically eg when switching from drain to
maint.

Note that the old actions are still supported for users running scripts.