Removing Manager and improve NewConfiguration typesafety

[abjeni]

This is a prototype of the different design that uses a configuration in place of a manager.
The raw configuration is now a struct that points to a raw manager as well as the slice of raw nodes.
The NewConfiguration method takes a set of manager options instead of requiring a manager.
RawManager should be made private since it is no longer used by the generated files.
Sub configurations are created with a SubConfiguration method, without manager options.
There is no main configuration, the sub configurations all belong to the same raw manager.
The Close method is used to close a configuration along with all of its sub configurations.

Notes

• NewConfiguration should only accept node map and node list.
• mgr_test.go and config_test.go uses RawManager directly outside of the gorums package.
• the storage example benefits from having a manager.
• tests/config/config_test.go need a way to create an empty configuration.

Fixes #192

[deepsource-io]

Here's the code health analysis summary for commits ca1cee9..125b0cf. View details on DeepSource ↗.

Analysis Summary

Analyzer	Status	Summary	Link
Go	❌ Failure	❗ 19 occurences introduced	View Check ↗
Shell	✅ Success		View Check ↗

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