Improve async cache

[notaphplover]

Description

This PR updates the resolver to update the cache of an async result to its resolved value once the promise is fullfilled

Related Issue

#1320

Motivation and Context

See #1320 for more details

How Has This Been Tested?

Code compiles and tests passses successfully

Types of changes

• Updated docs / Refactor code / Added a tests case (non-breaking change)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have updated the changelog.

[notaphplover]

@tonyhallett This is my proposal for improving the cache system on singleton scope :), I'll add tests and update docs later

[tonyhallett]

Looks good. If we go with my scope abstraction then the only change would need to be to the SingletonScope

class SingletonScope<T> implements interfaces.Scope<T>{
  get(binding:interfaces.Binding<T>,_: interfaces.Request): Promise<T> | T | null {
    if(binding.activated) {
      return binding.cache;
    }
    return null;
  }
  set(binding:interfaces.Binding<T>,_:interfaces.Request,resolved:T|Promise<T>):T | Promise<T> {
      binding.cache = resolved;
      binding.activated = true;

      if (isPromise(resolved)) {
          resolved = resolved.catch((ex) => {
              // allow binding to retry in future
              binding.cache = null;
              binding.activated = false;

              throw ex;
          });
      }
      return resolved;
  }

}

[PodaruDragos]

LGTM !