TMultiplexedProcessor should allow registering default processor called if no service name is present

The original intention of this story was to allow an older client to connect to a multiplexed server and continue to work by having the multiplexed processor handle any request that does not contain a service name and colon prefix. Given that multiplex is just a decorated protocol where the service name is prepended to the call name, it should be possible to connect a standard binary protocol client to a multiplex protocol server that treats a non-multiplex call (one without the colon separator) as a backwards-compatible call so older clients can continue to connect to newer servers.