MINOR: event_hdl: add global tunables

MINOR: event_hdl: add global tunables

The local variable "event_hdl_async_max_notif_at_once" which was
introduced with the event_hdl API was left as is but with a TODO note
telling that we should make it a global tunable.

Well, we're doing this now. To prepare for upcoming tunables related to
event_hdl API, we add a dedicated struct named event_hdl_tune which is
globally exposed through the event_hdl header file so that it may be used
from everywhere. The struct is automatically initialized in
event_hdl_init() according to defaults.h.

"event_hdl_async_max_notif_at_once" now becomes
"event_hdl_tune.max_events_at_once" with it's dedicated
configuation keyword: "tune.events.max-events-at-once".

We're also taking this opportunity to raise the default value from 10
to 100 since it's seems quite reasonnable given existing async event_hdl
users.

The documentation was updated accordingly.