Allow factories to customize their generated Providers

[copybara-service]

Allow factories to customize their generated Providers

This enables a few optimizations

• A number of implementations can avoid InternalContext overheads addressing High memory consumption to obtain a Singleton instance via Injector or Provider #1802
  • ConstantBindings, one time bindings like Initializable and @Singleton bound values, and a number of multibinder bindings in particular.
• We can leverage the cache within BindingImpl to share provider instances.

The tricky part is about multibindings, which have complex multi-phase initialization semantics, so we need to be defensive about makeProvider getting called prior to being initialized. To help manage this I

• reordered the LookupProcessor to run after multibinders.
  • The comment justifying its current location in initialization is obsolete since multibinders were rewritten to be 'native'.
  • This ensures lookups get optimized providers!
• made the SyntheticProviderFactory allocate its delegate lazily.
• make OptionalBinder explicitly initialize their delegate bindings and also be defensive against recursive and duplicate initialization. This is necessary to force initialization to be correct when our getExistingBinding query ends up delegating to another multibinder.
• be defensive about initialization failing to complete due to errors