ChoreService should be LimitedPrivate(COPROC)

ChoreService is marked Public and is in hbase-common but is not part of our public client API. We aren't providing a library for scheduling chores and such; we are providing a client library for accessing remote HBase service. As part of the server framework, where it could be useful for coprocessors, it makes more sense to expose it as supported for that purpose. I believe this was the intent of marking it Public. However I argue it should be LimitedPrivate(COPROC) instead, which is the appropriate classification for server internal components we want to expose to plug ins.