Consolidate Channel and MultiChannel code and add new features. Fixes #238

[plorenz]

• merges MultiChannel into Channel interface, removing the separate
  multi.go implementation and priority.go
• replaces UnderlayHandler god-interface with focused interfaces:
  Senders, MessageSourceProvider, UnderlayEventListener, and DialPolicy
• replaces GroupedUnderlayFactory with DialPolicy interface;
  adds BackoffDialPolicy with exponential backoff, short-lived
  connection detection, MinDialInterval, and LastDialTime()
• replaces UnderlayConstraints helper with Config.Constraints map
  and automatic constraint enforcement with CAS-guarded re-check
• adds UnderlayEventListener callbacks with Channel parameter so
  consumers can track underlay state changes
• adds Config.UnderlayEventListeners for consumer listener registration
• adds Config.ConstraintStartupDelay for delayed constraint application
• adds NewSingleChannelWithUnderlay for listener-side callers with
  bare underlays
• adds MessageQueue, MakeSource1/2/3, and RetryToQueues helpers to
  reduce priority sender boilerplate
• adds TypedBinding, TypedBindHandler, and TypedReceiveHandler generics
  for type-safe senders access during binding and message handling
• consolidates binding code into bind.go; handler types into handler.go
• adds comprehensive test suite (72+ tests across 10 files)
• adds godoc comments to all exported symbols
• clears waiters on channel close to release map entries eagerly
• fixes nil dialPolicy panic in applyConstraints
• fixes deferred re-check race in constraint enforcement