Solve legacy problem with clear operation (coredump with running threads)

Modified the clear operation. Now is 'conditional', so, when is unsafe,
it will be ignored. A message on http interface advice about trying
later because some running threads could be ongoing (check test
summary to confirm). Sometimes, it could be due to stuck external
procedures. The user is responsible to kill those stuck procedures
or guarantee they are timedout (it is recommended to wrap procedures
into safe containers (subshells) if you don't trust too much those
procedures).

Normally, step commands are not so usual. ADML FSM (finite state
machine) is better used through REST API from other controllers
with their own FSM and interface endpoints (no need for ADML
dedicated command-step threads).