Separate thrift handler for history service (cadence-workflow#3646)

This is purely a mechanical refactoring that introduces additional ThriftHandler wrapper. For now it only forwards request to the original handler. Later on this will become a point for conversion between internal types and thrift types. Underlying handler will be independent of rpc protocol types.
- Created Handler interface for history service
- Previous Handler struct renamed to handlerImpl
- Created ThriftHandler wrapper on top of Handler interface. At this point it only forward requests as is to underlying handler.
- Created Mock for Handler interface.
- Covered ThriftHandler with units tests. Test that requests are forwarded to underlying handler and response is returned as is.
- Replaced handler registration. Instead of registering handlerImpl directly, wrap it with ThriftHandler and register it instead.