* Rwandan Hutu FDLR rebels a priority after defeat of M23 in Congo
* Rebel group at heart of 20 years of instability in region
* Civilians wary after previous offensives failed to defeat rebels
By Peter Jones
TONGO, Democratic Republic of Congo, April 8 (Reuters) - Twenty years after the genocide in Rwanda, a rebel group founded by ethnic extremists who took part in that slaughter still...