private void dropTarget(Entry entry, Spec key) {
        entry.close();
        targets.remove(key);
    }
This method will return a target for the given address. If a target does not currently exist for the given
address, it will be created and added to the internal map. Each target is also reference counted so that an
active target is never expired.

@param orb     The supervisor to use to connect to the target.
@param address The address to resolve to a target.
@return A target for the given address.
messagebus/src/main/java/com/yahoo/messagebus/network/rpc/RPCTargetPool.java