Prevent target resource removal on removal of source

[alexsorkin]

Is your feature request related to a problem? Please describe.
The PR Delete replicated keys when source Secret/Resource is deleted has created a problem for my case. I need the target to be preserved in the event of source removal.
The target (configmap) was pre-existing, and should be left in place after replicator source removal.

Describe the solution you'd like
New push method annotation, "preserve-target-on-removal" or "disable-cascade-removal"

Describe alternatives you've considered
The pre-fix implementation was fine with me.
Also the PR StripAnnotations may give an implicit solution, but its not obvious.

Additional context
Assume, you are patching pre-existing coredns configmap..
The configmap should be preserved in the event of replication source removal.
The cluster becomes dysfunctional using current implementation.