MEDIUM: task: make notification_* API thread safe by default

MEDIUM: task: make notification_* API thread safe by default

Some notification_* functions were not thread safe by default as they
assumed only one producer would emit events for registered tasks.

While this suited well with the Lua sockets use-case, this proved to
be a limitation with some other event sources (ie: lua Queue class)

instead of having to deal with both the non thread safe and thread
safe variants (_mt suffix), which is error prone, let's make the
entire API thread safe regarding the event list.

Pruning functions still require that only one thread executes them,
with Lua this is always the case because there is one cleanup list
per context.