Introduce AbstractAsyncSelector - #6097

Motivation:

AbstractEndpointSelector is a useful abstract which allows users to select and Endpoint asynchronously only by implementing AbstractEndpointSelector#selectNow. However, the current implementation has limitations in that 1) Only Endpoint can be selected 2) It is difficult to customize the timing when the pending futures must be reconsidered for completion.

While working on the introduction of an XdsPreprocessor #6096, I found it is useful to be able to select a value asynchronously in the following locations:

1. When a request is executed, the request must wait until a ListenerSnapshot is fetched
   

   armeria/xds/src/main/java/com/linecorp/armeria/xds/client/endpoint/XdsPreprocessor.java

   Line 60 in 6bbc2ac

   public O execute(PreClient<I, O> delegate, PreClientRequestContext ctx, I req) throws Exception {

2. The XdsLoadBalancer which selects an endpoint must be refreshed either when the encompassed EndpointGroup is updated or when the local cluster is updated.
   

   armeria/xds/src/main/java/com/linecorp/armeria/xds/client/endpoint/UpdatableLoadBalancer.java

   Line 110 in 6bbc2ac

   endpointSelector.refresh();

For ease of implementation and maintainability, I propose that AbstractAsyncSelector is abstracted out of AbstractEndpointSelector. Since it is unclear whether this abstraction is useful for other situations, the class is located in the internal package.

Modifications:

• Abstracted out selection related logic from AbstractEndpointSelector to AbstractAsyncSelector
  • Now AbstractEndpointSelector contains an internal AbstractAsyncSelector implementation
• Moved SelectionTimeoutTest#select_unlimited to the internal package since it relies on visibility of ListenableFuture#timeoutFuture

Result:

• Users can implement async selection logic easily