Created ISyncBusConfiguration

Previously, SyncMessageBus required a full IBusConfiguration, which
has lots of methods about threading, etc. However, if you want a
purely Sync bus that never uses multithreading, you probably don't
want to create all those threads and dispatchers. This change allows
you to call `new SyncMessageBus(new SyncBusConfiguration())`, and
get an entirely multithreading-free Bus.