Improve typing support for services

[wayneparrott]

Problem
When working with Services in TypeScript the type definitions use semi-ambiguous Message[T] rather than the more precise MessageType. @mattrichard implementation of action typing demonstrate how to create unambiguous typing definitions. A similar technique could be used to improve the typing definitions of Services and Clients.

Proposal
This enhancement request proposes to apply the generics patterns used in the action typings to improve the Service and Client and related callback and handler definitions. The changes should should be backward compatible.

Expected outcome
The results should be much more precise service and client typings that smart typescript editors (e.g., VSCode) can employ to assist developers create clean, unambiguous services and clients.