Fix -bootstrapping option

Option was broken when event network was extracted from wifi manager.
Also fixed connectivity prober lifetime. Existing code was able to call
callback test by bufferevent_setcb even after EventNetworkImpl was
destroyed.

Change-Id: I774f37a55f0126fe384317ab69f077fa7817e923
Reviewed-on: https://weave-review.googlesource.com/1432
Reviewed-by: Vitaly Buka <vitalybuka@google.com>