Swift: Server-side support for Multiplexing Services

The Swift library features TMultiplexedProtocol but not TMultiplexedProcessor, which is required for use server-side.

I've added TMultiplexedProcessor, following the same patterns as the other language libraries. I've marked this issue as Breaking-Change since it removes the associatedType from the TProcessor protocol. I've removed the associatedType from TProcessor otherwise it makes it impossible for TMultiplexedProcessor to implement TProcessor. By definition it should support processors of multiple services, not a single service.

I've added tests which require a change to Package.swift to run using swift test.