refactor: simplify NewConfig signature; remove NewConfiguration and Manager alias

[meling]

Part of #294.

Summary

Simplify the NewConfig signature from a variadic ...Option to an explicit (nodes NodeListOption, dialOptions ...DialOption), remove the now-unused NewConfiguration function and Configuration.Manager() method (replaced by the unexported mgr()), add Configuration.Close() as a convenience, and strip the Manager alias and NewManager / NewConfiguration forwarding functions from the generated static code.

Motivation

• The old variadic ...Option allowed mixing ServerOption, NodeListOption, and ManagerOption in one call, which required runtime type-switching and produced confusing errors.
• NewConfiguration(mgr, opt) required callers to construct a Manager explicitly, which is now internal.
• The Manager alias in generated code leaked the implementation detail into every generated package.
• Users can still continue create sub-configurations using Difference and Remove or Extend and existing configuration with more nodes (and use the same underlying outboundManager as the original configuration.)

Changes

• config.go:
  • Change NewConfig(opts ...Option) to NewConfig(nodes NodeListOption, dialOptions ...DialOption)
  • Remove NewConfiguration
  • Rename Configuration.Manager() to Configuration.mgr() (unexported)
  • Add Configuration.Close() error
  • Update Extend and Add to call c.mgr()
• cmd/protoc-gen-gorums/dev/aliases.go:
  • Remove Manager alias, NewManager, NewConfiguration forwarding functions
  • Update NewConfig forwarding signature
• cmd/protoc-gen-gorums/gengorums/template_static.go:
  • Mirror the same changes in the static code template
  • Remove Manager from reservedIdents

Files Changed

config.go, cmd/protoc-gen-gorums/dev/aliases.go, cmd/protoc-gen-gorums/gengorums/template_static.go