Add PartOf=replds.service to the multi-instance units

Should hopefully let dpkg restart all the instances of the service.

Fixes issue #1.
4 jobs for master in 1 minute and 48 seconds
Status Job ID Name Coverage
  Build Pkgsrc
passed #10002
build_pkgsrc:stretch

00:00:18

 
  Build Pkg
passed #10003
amd64
build_pkg:stretch:amd64

00:01:05

passed #10004
arm64
build_pkg:stretch:arm64

00:00:41

 
  Upload Pkg
passed #10005
upload_pkg

00:00:25