importers: improve import of secfixes db

Only commit db once per repo. This improves performance significantly.

Before:

real	8m 9.09s
user	3m 21.13s
sys	0m 40.63s

After:

real	0m 15.95s
user	0m 14.30s
sys	0m 0.46s

Also add optional arg to only import secfixes for a give repo. This is useful so we can import secfixes on MQTT events.