Concurrency issue in RpcProviderRegistryImpl.java

With gerrit https://git.opendaylight.org/gerrit/#/c/9093/ we unwittingly fixed a concurrency issue with the publicProxies variable. However, the same concurrency issues exist with rpcRouters as well.

Basically, we synchronize when we write the the map, but we do not synchronize when we read from it. This can lead to inconsistent behavior in this class.