Signed-off-by: Xin Zhuang [email protected]

Description: Refactor config provider to:

• Move creation of Config implementation from x-ConfigProviders to x-Subscriptions, so that we can
  share one Config implementation across workers in a DeltaConfigSubscriptionInstance and its associated providers, and share one Config impl across all works and all providers in a ConfigSubscriptionInstance and its associated providers.
• Remove DeltaMutableConfigProviderBase, and MutableConfigProviderBase as now their functionality is covered by the DeltaConfigSubscriptionInstance/ConfigSubscriptionInstance impl.
• Adjusted tests.

Risk Level: LOW (no production code)
Testing: unit tests
Docs Changes: N/A
Release Notes: N/A