Fix gazebo8 warnings part 1: replace Events::Disconnect* with pointer reset (lunar-devel)

[j-rivero]

{ port of pull request #623 }
Alternative to #613 split into many parts.

sed -i \
  -e 's@[gazebo:]*event::Events::Disconnect[A-Za-z]*(\([^)]*\));@\1.reset();@' \
  $(grep -rlI Events::Disconnect .)

There were 983 warnings for #542; this should reduce that.