fix(java): stop Show Usages waiting seconds on the configuration index after a change

After any Java change, the configuration index was rebuilt from scratch on the next Show
Usages, resolving every dependency's Spring metadata (thousands of Spring Boot properties)
to library members again, and Show Usages waited for it: 15-20 seconds in a Spring Boot +
MyBatis project.

- The Java extension (1.8.0) keeps each dependency JAR's metadata entries across rebuilds
  until the JAR's timestamp/size or the project's classpath changes.
- Show Usages shows the Java usages as soon as they are found and adds the configuration
  ones when the index takes longer.

Closes #1898

java/java-usages/CONFIGURATION.md

@@ -78,7 +78,13 @@ changes; a fingerprint of working copies with unsaved changes covers edits befor
 they are saved. Opening or closing an editor, reconciling, building, and non-Java
 resource changes (configuration files are parsed by the client; metadata files are
 checked by timestamp) keep it. This is **snapshot invalidation**, not a fully
-incremental Java semantic index.
+incremental Java semantic index. A rebuild reuses each dependency JAR's metadata
+associations (up to 100,000 in all) while the JAR's timestamp and size and the project's
+classpath are unchanged: resolving a Spring Boot JAR's thousands of properties to library
+members takes seconds, and a source change cannot affect them.
+
+Find Usages shows the Java usages as soon as they are found, then adds the configuration
+ones when a rebuild of the snapshot takes longer.
 
 In YAML/properties, ⌘/Ctrl-hover underlines the whole key, hyphens and dots
 included, and doesn't open the documentation popup. Hover and Go to Definition share