MINOR: event_hdl: pause/resume for subscriptions

MINOR: event_hdl: pause/resume for subscriptions

While working on event handling from lua, the need for a pause/resume
function to temporarily disable a subscription was raised.

We solve this by introducing the EHDL_SUB_F_PAUSED flag for
subscriptions.

The flag is set via _pause() and cleared via _resume(), and it is
checked prior to notifying the subscription in publish function.

Pause and Resume functions are also available for via lookups for
identified subscriptions.

If 68e692da0 ("MINOR: event_hdl: add event handler base api")
is being backported, then this commit should be backported with it.