Previously the datacenter of the gateway was the key identifier, now it
is the datacenter and partition.
When dialing services in other partitions or datacenters we now watch
the appropriate partition.
This method was accidentally re-introduced in an earlier rebase. It was
removed in ed1082510dc80523b1f2a3a740fa5a13c77594f9 as part of the tproxy work.
There is no interaction between these handlers, so splitting them into separate files
makes it easier to discover the full implementation of each kindHandler.
freddygv