introduce context->event_ops

Signed-off-by: Alejandro Mery <alejandro.mery@opensynergy.com>
libev: mutilate and prepare for context->event_ops usage

Signed-off-by: Alejandro Mery <alejandro.mery@opensynergy.com>
libev: rename to lib/event-libev.c

Signed-off-by: Alejandro Mery <alejandro.mery@opensynergy.com>