proc_mgr: introducing event driven thread option

[uLipe]

for step process manager, which is triggered by FIFO samples incoming. This allows to keep the node execution automatic without polling manually the process manager.

To enable just hit the option CONFIG_STEP_PROC_MGR_EVENT_DRIVEN in the menuconfig.

One limitation, periodic polling cannot be used if event driven process manager is enabled, and it is automatically disabled if the event thread mechanism is used.

I also kept the compatibillity of the manually polling to avoid unit tests breaking.

[uLipe]

@microbuilder PTAL

[uLipe]

@microbuilder PTAL again.