Added the column uniq_ruptures to the table source_info

Now the table source_info contains three kind of rupture counters:

num_ruptures = total number of ruptures that could be generated
uniq_ruptures = number of unique ruptures occurring in an event_based calculation
occ_ruptures =  total number of ruptures occurring in an event_based calculation

In generale occ_ruptures >= uniq_ruptures since the same rupture can occur more than once in an event based
calculation. In a classical calculation uniq_ruptures e occ_ruptures are NULL, only num_ruptures is stored.

Tests running here: https://ci.openquake.org/job/zdevel_oq-engine/572