This patch adds two support functions for adding reconnection callbacks
and invoking such callbacks. The concept of reconnection is simple: stop
using current connection for for new queries to the server without
actually going offline.

In several places, we were creating a new timer and assigning it
to the tev variable, but then we were checking for NULL from the
te variable (which, incidentally, is guaranteed never to be NULL
in this situation)

https://fedorahosted.org/sssd/ticket/523